Method and system for material characterization of optical surface defects

Abstract

Methods and systems for detecting and classifying defects based on dark field scatter phase from a sample are described herein. In some embodiments, throughput is increased by detecting and classifying defects with the same optical system. In one aspect, defects are classified based on measured relative phase of scattered light collected from at least two spatially distinct locations in a collection pupil. If there is a phase difference between light transmitted through any two spatially distinct locations at a pupil plane, the phase difference is determined from the location of interference fringes in an imaging plane. The measured phase difference is indicative of the material composition of the measured sample. In another aspect, an inspection system includes a programmable pupil aperture device configured to sample a pupil at different programmable locations in a collection pupil.

Description

[0001] Cross Reference to Related Applications

[0002] This patent application claims priority under 35 U.S.C. § 119 to U.S. Provisional Patent Application No. 62 / 871,872, entitled “Method Of Defect Material Characterization Using Optical Microscope,” filed on July 9, 2019, the subject matter of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0003] The described embodiments relate to systems for surface inspection, and more particularly to semiconductor wafer inspection modalities. BACKGROUND

[0004] Semiconductor devices, such as logic and memory devices, are typically fabricated by a sequence of processing steps applied to a substrate or wafer. Various features of the semiconductor devices and multiple levels of structures are formed by these processing steps. For example, photolithography is one semiconductor fabrication process that involves creating patterns on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices can be fabricated on a single semiconductor wafer, and then separated into individual semiconductor devices.

[0005] The semiconductor fabrication environment is carefully controlled to minimize wafer contamination. Unwanted material particles can interfere with the fabrication process, degrade the performance of the fabricated devices, or both. As semiconductor design rules continue to evolve, the requirements for uniformity and cleanliness of the substrates continue to increase. The number of defects allowed and the maximum allowed particle size scale with the size of the devices in fabrication.

[0006] Generally, an inspection system detects any type of defects on a specimen at any point in the production process. In some examples, an inspection system is typically used to locate defects on a substrate prior to processing to ensure that the substrate is suitable for continued fabrication or to identify defect sites on a wafer prior to production. One such inspection system is an optical surface inspection system that illuminates and inspects a wafer surface for non-desired particles. Optical surface inspection systems are typically high-throughput systems that locate defects and generate a map of the defects located on each inspected wafer.

[0007] Additionally, it is often desirable to classify defects by material type, structural properties, etc. Typically, a separate defect review tool is used to perform the defect classification task. In some instances, defects are located by an optical inspection tool. The inspected wafer and defect location map are transferred to a defect review tool. The defect review tool performs detailed analysis of one or more of the defect locations identified by the optical inspection system to classify the defect at each location.

[0008] Defects are typically classified by material composition. In one instance, knowledge of the defect material composition enables an operator to determine an appropriate cleaning procedure to remove the defect particle from the wafer. In another instance, knowledge of the defect material composition indicates a source of wafer contamination, thus enabling an operator to take action to stop further wafer contamination.

[0009] Conventional defect classification tools and techniques are limited in sensitivity to small particle size, throughput, or both. For example, energy dispersive X-ray spectroscopy (EDX) provides defect material analysis capabilities with high sensitivity for some materials, but not for other materials such as inorganic compounds or organic particles. Additionally, the throughput of EDX is insufficient to enable cost-effective defect classification in a semiconductor fabrication facility.

[0010] To increase throughput, it is desirable to perform at least some of the defect classification tasks with the same optical inspection tool used to detect the defect locations. In particular, performing defect composition analysis with an optical inspection tool offers significant advantages over conventional analysis tools that employ EDX or secondary ion mass spectroscopy (SIMS) techniques. Performing defect classification with an optical inspection tool eliminates the need to transfer the wafer and defect map to another tool for defect classification. Additionally, defect classification results are available immediately from the optical inspection tool. In a typical semiconductor device manufacturing process, this reduces turnaround time from hours to minutes. Additionally, defect classification with an optical inspection tool is non-destructive; analysis is performed without the need to destroy the sample, remove material from the sample, etc.

[0011] U.S. Patent Publication No. 2018 / 0188188 to Zhao et al. and assigned to KLA Corporation describes various optical inspection systems that detect defects at high throughput with sufficient sensitivity to small particle sizes and classify the defects, the contents of which are incorporated herein by reference in their entirety. In particular, Zhao describes optical inspection systems that employ phase-shifted phase-contrast imaging techniques to classify defects. Phase-shifted phase-contrast techniques require spatial separation of specularly reflected light and scattered light at a collection pupil plane of the optical system to introduce a relative phase shift between the specularly reflected light and the scattered light. To achieve this spatial separation, the distribution of the illumination beam is restricted to a selected location within the pupil plane of the objective lens. This limits the number of photons provided to the wafer by the illumination source, which in turn limits the sensitivity of the optical inspection system.

[0012] As semiconductor design rules continue to evolve, the minimum particle size that must be detected by a surface inspection system continues to shrink in size. In addition, it can be desirable to classify detected defects with the same optical tool used to detect the defects. The steady decrease in size of fabricated features and the need to simultaneously detect and classify defects presents challenges to the sensitivity and throughput of inspection systems.

[0013] Improvements to optical surface inspection systems are desired to detect defects in the inspection path of an illumination light spot on a wafer surface with greater sensitivity and throughput and to classify the defects. SUMMARY

[0014] Described herein are methods and systems for detecting and classifying defects based on the phase of dark-field scatter from a sample. In some embodiments, throughput is increased by detecting defects and classifying the defects with the same optical system (i.e., defect inspection and defect re-inspection performed by the same optical tool). In other embodiments, optical inspection and optical re-inspection tools are enhanced by incorporating the techniques described herein.

[0015] In one aspect, defects are classified based on the measured relative phase of scattered light collected from at least two spatially disparate locations within a focusing pupil. Specifically, the defect classification is based on the measured relative phase of the scattered light with respect to a given illumination angle. Therefore, the techniques described herein are implemented only in the collecting optics of the inspection system, resulting in a simple and cost-effective optical inspection / defect re-inspection system. Scattered light is collected from at least two spatially disparate locations within the focusing pupil, while residual light is blocked. Under these conditions, a well-defined interference pattern is formed at the image plane of the detector's photosensitive surface. If a phase difference exists between the light transmitted through the two spatially disparate locations at the pupil plane, the phase difference is determined based on the position of the interference fringes in the image plane. The measured phase difference indicates the material composition of the sample being measured.

[0016] In another embodiment, the difference between the measured phase difference and a known phase difference value associated with the sample is determined as a correction value. In some embodiments, the material being measured is a known material with known material properties and phase response. In these embodiments, the difference between the phase difference measured by the inspection system and the known phase difference indicates a systematic error in the measurement system, such as optical aberrations, measurement electronics errors, etc. The correction value is stored in a memory. Subsequent phase difference measurements performed by the system are corrected by the stored correction value to compensate for the systematic error present in the phase difference measurement.

[0017] In another instance, the Fast Fourier Transform (FFT) algorithm is used to extract phase difference information from the interference pattern present in the detected image. The FFT algorithm provides excellent noise removal and is computationally efficient. In other instances, the phase difference between light scattered from two different locations in the pupil plane is determined by iterative fitting of a measured physical model with the measured interference pattern.

[0018] In another aspect, the inspection system includes a programmable pupil aperture device configured to sample the pupil at different programmable positions within the focusing pupil. In this way, the positioning of each sampling position in the pupil plane is controlled for each phase difference measurement.

[0019] The foregoing is a summary of the invention and therefore necessarily contains simplified, generalized, and omitted details; thus, those skilled in the art will understand that the summary is illustrative only and not in any way limiting. Other aspects, inventive features, and advantages of the apparatus and / or process described herein will become apparent from the non-limiting specific embodiments set forth herein. Attached Figure Description

[0020] Figure 1is a simplified diagram illustrating one embodiment of an inspection system configured to measure phase differences between scattered light collected from a sample at different locations in a collection pupil.

[0021] Figure 2 is a simplified diagram illustrating a wafer 110 illuminated by an illumination beam.

[0022] Figure 3 is a diagram illustrating a mask that blocks all light collected at the NA except for light transmitted through an aperture.

[0023] Figure 4 is a plot depicting a simulation of an interference pattern at an image plane resulting from interference of light scattered from a material and transmitted through an aperture at a different location in a collection pupil.

[0024] Figure 5 is a plot depicting a simulation of an interference pattern at an image plane resulting from interference of light scattered from a material different from the material depicted in Figure 4 and transmitted through an aperture identical to the aperture depicted in Figure 4 .

[0025] Figure 6 is a plot depicting a simulation of an interference pattern at an image plane resulting from interference of light scattered from a material different from the material depicted in Figure 4 and 5 and transmitted through an aperture identical to the aperture depicted in Figure 4 .

[0026] Figure 7 is a diagram illustrating pupil apertures in an embodiment symmetrically located around the center of the pupil in the x-direction.

[0027] Figure 8 is a diagram illustrating pupil apertures in another embodiment symmetrically located around the center of the pupil in the x-direction.

[0028] Figure 9 is a diagram illustrating pupil apertures in an embodiment symmetrically located around the center of the pupil in the y-direction.

[0029] Figure 10 is a diagram illustrating pupil apertures in another embodiment symmetrically located around the center of the pupil in the y-direction.

[0030] Figure 11 is a simplified diagram illustrating a programmable pupil mask device in an embodiment.

[0031] Figure 12 FIG. 3 is a simplified schematic illustrating a programmable pupil mask apparatus in another embodiment.

[0032] Figure 13 A flowchart illustrating an exemplary method 200 that can be used to measure phase differences between scattered light collected from a sample at different locations in a collection pupil is illustrated. DETAILED DESCRIPTION

[0033] Reference will now be made in detail to background examples and some embodiments of the application, examples of which are illustrated in the accompanying drawings.

[0034] Methods and systems for detecting and classifying defects based on the phase of dark field scatter from a sample are described herein. In some embodiments, nanoscale defect particles are detected and classified according to the methods and systems described herein. The methods and systems presented herein for detecting and classifying defects are non-destructive and can be integrated into an optical inspection tool, a defect review tool, or an integrated optical inspection / defect review tool. In some embodiments, throughput is increased by detecting and classifying defects with the same optical system (i.e., defect inspection and defect review performed by the same optical tool). In other embodiments, optical inspection and optical review tools are enhanced by incorporating the techniques described herein.

[0035] Generally, light scattering from a defect depends on many properties of the defect. For example, in addition to material properties such as complex refractive index described by refractive index n and extinction coefficient k, geometric properties such as defect shape and size also affect light scattering. The values of material parameters such as n and k are indicative of material composition. However, material parameters such as n and k cannot be determined directly from simple scattered light intensity measurements because they are not effectively decoupled from geometric properties in light intensity measurements.

[0036] In one aspect, defects are classified based on the measured relative phase of scattered light collected from at least two spatially distinct locations in a collection pupil. In particular, defect classification is based on the measured relative phase of scattered light for a given illumination angle. Thus, the techniques described herein are implemented only in the collection optics of an inspection system; resulting in a simple and cost-effective optical inspection / defect review system.

[0037] In one example, defect particles are classified as high-k metals or low-k transparent dielectric materials based on the measured relative phase of scattered light at different locations in the pupil plane.

[0038] In some examples, values of material properties of the defect, such as n and k, are determined based on the phase of light scattered from the defect. In particular, the material properties are determined based on measured phase differences of scattered light at different locations in the pupil plane. In these examples, the defect is classified based on the determined material properties of the defect.

[0039] Figure 1 is a simplified schematic diagram of one embodiment of a surface inspection system 100 having inspection and classification functionality as described herein. The surface inspection system 100 is provided by way of non-limiting example. In general, any optical microscope or inspection system that images scattered light on a sensor to form an image of a defect as described herein is suitable for implementing the inspection and classification functionality described herein. Such an optical microscope or inspection subsystem can be implemented as part of a benchtop analysis tool or as part of an automated system for defect inspection, defect review, or both.

[0040] Some optical components of the system have been omitted for simplicity. By way of example, folding mirrors, polarizers, beam shaping optics, additional light sources, additional collectors, and additional detectors can also be included. All such variations are within the scope of the application described herein. The inspection systems described herein can be used to inspect unpatterned wafers as well as patterned wafers.

[0041] As Figure 1 illustrated in Figure 1 , the illumination source 101 generates an illumination beam 102 directed toward the wafer 110. In the embodiment depicted in Figure 1 , the focusing optics 103 focus the illumination light 102 onto the wafer 110 over a measurement spot 104. However, in general, any suitable illumination optics can be employed to provide the illumination light 102 onto the wafer 110 with a desired measurement spot size. In some embodiments, one or more beam shaping elements are included in the illumination optical path (i.e., the optical path between the illumination source 101 and the wafer 110) to form a desired beam profile. Exemplary beam profiles include a Gaussian beam shape, a ring beam shape, a flat top beam shape, etc. Typical measurement spot sizes include measurement spots having lengths from as small as one micron to as large as five hundred microns characterized by a dimension across the longest extent of the measurement spot.

[0042] In some embodiments, one or more polarizer elements are located in the illumination optical path to polarize the illumination light in a desired manner. Exemplary polarizations include linear polarization, elliptical polarization, circular polarization, or no polarization.

[0043] As Figure 1As depicted, the illumination subsystem provides illumination 102 to the surface of wafer 110 at a tilt angle. However, generally, the illumination subsystem can be configured to guide the beam to the sample at a normal incidence angle. Typical incidence angles range from zero degrees (normal incidence) to eighty degrees with respect to normal incidence. In some embodiments, system 100 can be configured to guide multiple beams to the sample at different incidence angles (e.g., tilt angle and normal incidence angle). Multiple beams can be guided to the sample substantially simultaneously or sequentially.

[0044] By way of example, illumination source 101 may include a laser, diode laser, helium-neon laser, argon laser, solid-state laser, diode-pumped solid-state (DPSS) laser, xenon arc lamp, gas discharge lamp, and LED array or incandescent lamp. The light source may be configured to emit near-monochromatic light or broadband light. In some embodiments, the illumination subsystem is configured to direct light with a relatively narrow wavelength band to the sample (e.g., near-monochromatic light or light with a wavelength range less than about 20 nm, less than about 10 nm, less than about 5 nm, or even less than about 2 nm). Therefore, if the light source is a broadband source, the illumination subsystem may also include one or more spectral filters that limit the wavelength of the light directed to the sample. The one or more spectral filters may be bandpass filters and / or edge filters and / or notch filters. In some instances, the wavelength of light incident on wafer 110 includes any subset of wavelengths from infrared to extreme ultraviolet. Generally, illumination source 101 emits radiation at any desired wavelength or wavelength range within the optical wavelength range.

[0045] exist Figure 1 In the embodiments depicted, the illumination source 101 is configured to control the optical power of the illumination beam 102 according to a command signal 134 received from the computing system 140. In one embodiment, the illumination source 101 dynamically adjusts the illumination power during surface inspection scanning.

[0046] exist Figure 1 In the embodiment illustrated herein, the wafer positioning system 125 moves the wafer 110 beneath the measuring spot 104. The wafer positioning system 125 includes a wafer chuck 109, a motion controller 123, a rotary stage 121, and a translational stage 122. The wafer 110 is supported on the wafer chuck 109. Figure 2 As illustrated in the diagram, wafer 110 is positioned such that its geometric center 150 is substantially aligned with the axis of rotation of the rotating stage 121. In this manner, the rotating stage 121 causes wafer 110 to spin around its geometric center at a predetermined angular velocity ω within acceptable tolerances. Additionally, the translation stage 122 causes wafer 110 to move at a predetermined speed V. TThe wafer 110 is translated in a direction approximately perpendicular to the axis of rotation of the rotary stage 121. The motion controller 123 coordinates the spin of the wafer 110 via the rotary stage 121 and the translation of the wafer 110 via the translation stage 122 to achieve the desired scanning motion of the wafer 110 within the inspection system 100.

[0047] In the exemplary operating scenario, the inspection begins with a measurement spot 104 located at the geometric center 150 of wafer 110, and then wafer 110 is rotated and translated until the measurement spot 104 reaches the outer perimeter of wafer 110 (i.e., when R equals the radius of wafer 110). Due to the coordinated movement of the rotating stage 121 and the translating stage 122, the trajectory of the point illuminated by the measurement spot 104 travels along a spiral path on the surface of wafer 110. The spiral path on the surface of wafer 110 is referred to as inspection track 127 (not shown in its entirety). Figure 2 A partial illustration of the Lieutenant General's demonstrative test track 127 is shown as TRACK. i .like Figure 2 As illustrated in the diagram, the measurement spot 104 is located at a distance R from the geometric center of the wafer 110, and the defect particle 126 is approaching the measurement spot 104. In some embodiments, the detection system 100 is capable of locating defect particles as small as 50 nanometers along the maximum width of the particle. In some embodiments, the inspection system 100 is capable of locating defect particles as small as 10 nanometers along the maximum width of the particle.

[0048] like Figure 1 As depicted, the inspection system 100 includes an imaging condenser objective 112 for imaging light 111 scattered and / or reflected from the wafer 110 at the measurement spot 104 within a collection angle range onto one or more wafer image planes (e.g., image plane 119) of a collection optics subsystem. The objective 112 is configured to collect dark-field scattered light. In some embodiments, the objective 112 captures scattered light having a numerical aperture (NA) of 0.1 to 0.99.

[0049] although Figure 1 The diagram illustrates the specific nominal orientation of the light-collecting objective 112, but it should be understood that the orientation of the light-collecting objective relative to the wafer surface may be appropriately arranged depending on (for example) the incident angle and / or topographic characteristics of the wafer.

[0050] In some embodiments, the collecting optical path (i.e., the optical path between wafer 110 and detector 120) includes one or more polarizer optical elements 113 to select light with desired polarization. In some embodiments, the one or more polarizer elements 113 comprise a simple polarizer. In some other embodiments, the one or more polarizer elements 113 comprise a phase plate combined with a polarizer. In some of these embodiments, the phase plate is designed to modify the polarization of the scattered light.

[0051] In some embodiments, the collecting optical path includes one or more pupil relay optics (e.g., pupil relay optics 115) to form one or more relayed pupil planes (e.g., pupil plane 106). This can be desirable to allow easy access to the collecting pupil plane of one or more light-modifying elements (e.g., masking elements) to control the amount of light collected from a specific region of the pupil as described herein. Alternatively, it is desirable to position all light-modifying elements (e.g., masking elements) that control the amount of light collected from a specific region of the pupil as described herein at or near a pupil plane. Figure 1 As depicted, the optical path for collecting light includes two pupil planes (e.g., pupil planes 105 and 106), and the pupil mask 114 is located at the pupil plane 106.

[0052] exist Figure 1 In the embodiment depicted, optical element 117 focuses the collected light 111 onto image plane 119, where the image is detected by detector 120.

[0053] Imaging detector 120 generally serves to convert the detected light into electrical signals indicative of a detected image of wafer 110 within the detected field of view. Generally, imaging detector 120 can include substantially any photodetector known in the art. However, particular detectors can be selected for use within one or more embodiments of the present disclosure based on the desired performance characteristics of the detector, the type of sample to be inspected, and the configuration of the illumination. Generally, detector 120 acquires image information in either a frame mode or a scan mode. In a scan mode, the image is collected while wafer 110 is moving. If the amount of light available for inspection is relatively low, efficiency-enhanced detectors such as time delay integration (TDI) cameras can be employed to increase the signal-to-noise ratio and throughput of the system. In another example, signal integration is employed to achieve sufficient SNR for phase measurement. Integration times can be selected from a few nanoseconds to a second, depending on the available signal. A disadvantage of long integration times is that the measurement time is long and susceptible to environmental disturbances such as machine vibration. Depending on the amount of light available for inspection and the type of inspection being performed, other detectors such as charge-coupled device (CCD) cameras, photodiode arrays, phototubes, and photomultiplier tubes (PMTs) or individual PMT / photodiodes with a scannable aperture in front of the detector can be used.

[0054] Imaging detector 120 can be implemented in various imaging modes such as bright field, dark field, and confocal. Various imaging modes such as bright field, dark field, and phase contrast can be implemented by using different apertures or Fourier filters. U.S. Patent Nos. 7,295,303 and 7,130,039, which are incorporated by reference herein, describe these imaging modes in further detail. In the depicted example, detector 120 produces a dark field image by imaging the scattered light collected at a large field angle. In another example, a pinhole matching the point of incidence 104 can be placed in front of the detector (e.g., detector 120) to produce a confocal image. These imaging modes are described in further detail in U.S. Patent No. 6,208,411, which is incorporated by reference herein. Additionally, various aspects of surface inspection system 100 are described in U.S. Patent Nos. 6,271,916 and 6,201,601, which are incorporated by reference herein.

[0055] In another aspect, computing system 140 is configured to determine the location of the defect in the scan path based on changes in the detected signal 131. Additionally, computing system 140 is configured to classify the defect based on its material properties as described herein.

[0056] In one aspect, the scattered light is collected from at least two spatially distinct locations in the collection pupil while blocking the remaining light. Under these conditions, a well-defined interference pattern is formed at the image plane at the light sensitive surface of the detector. If there is a phase difference between the light transmitted through the two spatially distinct locations at the pupil plane, the phase difference is determined from the position of the interference fringes in the imaging plane. The measured phase difference is indicative of the material composition of the measured sample.

[0057] In another aspect, the difference between the measured phase difference and a known phase difference value associated with the same sample is determined by the computing system 140 as a correction value. In some embodiments, the material under measurement (i.e., within the measurement spot 104) is a known material with known material properties and phase response. In these embodiments, the difference between the phase difference measured by the inspection system (e.g., the inspection system 100) and the known phase difference is indicative of systematic errors in the measurement system, e.g., optical aberrations, measurement electronics errors, etc. The correction value is stored in a memory (e.g., the memory 142). Subsequent phase difference measurements performed by the system are corrected by the stored correction value to compensate for the systematic errors present in the phase difference measurements. The correction value is valid for measurements performed by the inspection system using the mask arrangement used to perform the calibration measurement (i.e., the specific locations in the pupil sampled during the calibration measurement). Additional calibration measurements can be performed for different mask arrangements as described herein to determine a correction factor for each set of sampling locations.

[0058] As Figure 1 depicted in FIG. 1, the mask 114 is located at the pupil plane 106. Another option is that the mask 114 can be located at the pupil plane 105. Whether the mask is located at one or the other pupil plane is a matter of design preference and all alternatives are contemplated within the scope of this patent document. Figure 3 A graphical illustration of the mask 114 is depicted. As Figure 3 depicted in FIG. 1, the mask 114 blocks all the light collected at the NA except for the light transmitted through the apertures 151 and 152. In the embodiment depicted in FIG. 1, the mask 114 is a binary mask. In other embodiments, the mask 114 can be a phase mask. Figure 3 In the embodiment depicted in FIG. 1, the center of the aperture 151 is located at 0.358 NA x and 0.0 NA y (corresponding to an angle of incidence of 21 degrees at the wafer). The radius of the aperture 151 is 1 / 12 of the radius of the pupil 153. The center of the aperture 152 is located at 0.788 NA x and 0.0 NA y (corresponding to an angle of incidence of 52 degrees at the wafer). The radius of the aperture 152 is 1 / 12 of the radius of the pupil 153. Whether all the apertures are located at a single pupil plane or at different pupil planes is a matter of design preference and all alternatives are contemplated within the scope of this patent document.

[0059] Figure 4 A plot 160 depicting another simulation of an interference pattern at the image plane at the photosensitive surface of the detector 120, the interference pattern resulting from interference of light transmitted through the apertures 151 and 152. In this example, the detector 120 includes a 973 x 973 array of 70-micron square pixels, and the illumination light has a wavelength of 266 nanometers. Figure 4 A zoomed-in view of the image plane at the center of the measurement spot is illustrated. As Figure 4 Illustrated in

[0060] Figure 5 A plot 161 depicting another simulation of an interference pattern at the image plane at the photosensitive surface of the detector 120, the interference pattern resulting from interference of light transmitted through the apertures 151 and 152 from a different material than Figure 4 . Figure 5 A zoomed-in view of the image plane at the center of the measurement spot is illustrated. As Figure 5 Illustrated in A

[0061] Figure 6 A plot 162 depicting another simulation of an interference pattern at the image plane at the photosensitive surface of the detector 120, the interference pattern resulting from interference of light transmitted through the apertures 151 and 152 from another different material than Figure 4 . Figure 6 A zoomed-in view of the image plane at the center of the measurement spot is illustrated. As Figure 6 Illustrated in B

[0062] As Figure 5 and 6 ​​As depicted in FIG. 1, each measured material exhibits a phase difference between scattered light collected from the sample material and transmitted through aperture 151 and scattered light collected from the sample material and transmitted through aperture 152. Most importantly, the phase difference associated with each material is significantly different. For example, as depicted in FIG. 1, the phase difference associated with material 1 is significantly different than the phase difference associated with material 2. In fact, the phase difference associated with material 1 is approximately 180 degrees out of phase with the phase difference associated with material 2. In other words, the phase difference associated with material 1 is approximately 180 degrees out of phase with the phase difference associated with material 2. Figure 5 and 6 As depicted in FIG. 1, the phase difference associated with each material is represented by the difference between φ A and φ B The difference in phase difference, represented by the difference between φ A and φ B is approximately 0.6 times the spatial period of the interference fringes (i.e., approximately 200 degrees). Thus, the phase difference value (i.e., the relative phase between two different locations in the pupil plane corresponding to two different materials) is indicative of the difference in optical properties (e.g., n and k values) between the materials.

[0063] The specific locations and sizes of apertures 151 and 152 are provided by way of non-limiting example. In general, many different aperture sizes and locations can be considered within the scope of this patent document. For example, the size of each aperture in the pupil plane can be in the range from 0.01 NA to 0.3 NA.

[0064] In general, the phase difference values measured using a particular mask geometry do not uniquely identify the material composition of the measured sample, although this can be the case in some situations. In order to distinguish between materials with high confidence, phase difference values can be measured using several different mask geometries, i.e., measuring the phase difference associated with multiple sets of different locations in the pupil plane. If the number of different mask geometries is sufficiently large, a phase map in the pupil plane can be derived from the measured interference fringes associated with each of the different mask geometries. This phase map is then used to uniquely identify the material properties, e.g., n and k, of the measured sample. In one example, the values of the material parameters (e.g., n and k) are floated in a physical model of the material and an iterative fitting procedure is employed to estimate the values of the material parameters that optimally fit the measured phase map.

[0065] The inventors have discovered that useful material characterization information can be derived from phase difference measurements of a sample having as few as two different mask geometries. In some examples, phase difference measurements of a defective particle having two different mask geometries are sufficient to classify the measured particle as a metal (very large k value) or a non-metal (very small or zero k value) with greater than 90% accuracy.

[0066] In another aspect, the computing system 140 is configured to extract phase difference information from the interference pattern present in the image 131 detected by the detector 120. In some examples, a fast Fourier transform (FFT) algorithm is employed to determine the phase difference between light scattered from two different locations in the pupil plane based on the measured interference pattern. The FFT algorithm provides excellent noise rejection and is computationally efficient. In other examples, an iterative fit of a physical model of the measurement to the measured interference pattern is employed to determine the phase difference between light scattered from two different locations in the pupil plane. In this example, one or more parameters indicative of the phase difference are floated in the physical model and the values are estimated in an iterative manner.

[0067] In some embodiments, the aperture locations in the pupil plane are optimized to enhance the contrast between measured phase differences for different materials. For example, if the spacing between the apertures in the pupil plane is too large, the measured phase differences will look similar for many different materials. However, if the spacing between the apertures in the pupil plane is too small, the values of the measured phase differences will have a low signal-to-noise ratio. In many examples, the spacing between the apertures is informed by knowledge of the phase map associated with each material of interest. For example, if it is known that a large transition in phase difference occurs within a particular NA range, the spacing of the apertures is selected to span only the NA range within which the transition is known to occur. In some examples, the spatial spacing between the apertures in the pupil plane spans an NA range from 0.1 to 0.9.

[0068] In other embodiments, the aperture locations are selected to minimize measurement error induced by focus offset (i.e., focus error) of the inspection tool. The material characterization of phase difference through the collection pupil as described herein is relatively insensitive to focus offset. However, the sensitivity of the measurement of phase difference to focus offset depends on the location of the different apertures. In some examples, the apertures are symmetrically located around the center of the pupil to minimize the sensitivity of the measurement to focus offset. Figure 7 and 8 Different locations of the apertures 151 and 152 symmetric around the center of the pupil in the x-direction are depicted. Figure 9 and 10 Different locations of the apertures 151 and 152 symmetric around the center of the pupil in the y-direction are depicted.

[0069] In some embodiments, the size of the aperture located in the pupil plane is optimized to both enhance the contrast between the measured phase differences for different materials and minimize the time necessary for data collection, especially for particles smaller than 100 nanometers. If the aperture size is too small, then very little light is transmitted. This requires signal integration over long acquisition times to achieve sufficient signal-to-noise ratio. However, if the aperture size is too large, then it becomes difficult to distinguish the phase differences associated with different materials because light from too many parts of the pupil is measured. In some embodiments, the aperture size is in the range from 0.01 NA to 0.3 NA.

[0070] Although the calculation of the phase difference between two different locations in the pupil plane is discussed in the foregoing, in general, the phase difference can be calculated among more than two locations, e.g., three or more locations. Estimating the phase difference among more than two locations shortens the acquisition time, but requires a more computationally complex phase determination.

[0071] Characterizing the material property based on the measured phase differences associated with different locations in the pupil plane relies on the non-uniformity of the phase of the scattered light in the pupil plane. However, in addition, the intensity of the light scattering in the pupil plane can also be very non-uniform. If the intensity difference is sufficiently large, then the contrast of the interference fringes is significantly reduced and the resolution of the phase measurement is affected. In some embodiments, this limitation is overcome by attenuating the transmitted light from one aperture relative to another. In some embodiments, a neutral density filter, a smaller aperture size, or a combination of both are employed to attenuate the intensity of the transmitted light passing through one aperture relative to another. Figure 1 A neutral density filter 118 at the pupil plane 106 is depicted that spans aperture 152 but not aperture 151. In this way, the intensity of the light transmitted through aperture 152 that reaches the detector 120 is attenuated relative to the intensity of the light transmitted through aperture 151 that reaches the detector 120.

[0072] As described in the foregoing, in some instances, two or more different locations in the sampling locations in the pupil are needed to classify the defect. In addition, the optimal sampling locations in the pupil vary depending on the material under consideration.

[0073] In another aspect, the inspection system includes a programmable pupil aperture device configured to sample the pupil at different locations under the control of the computing system 140. In this way, the computing system 140 measures the positioning of each sampling location in the pupil plane for each phase difference.

[0074] Figure 11 A programmable pupil mask device 170 is depicted in one embodiment. As Figure 11As depicted in FIG. 1, programmable pupil mask device 170 includes mask element 171 and mask element 172. Mask element 171 includes optical elements 171A and 171B that block light collected in the pupil. Optical elements 171A and 171B are fixed relative to each other and fixed in their positions within the pupil. Optical elements 171A and 171B are spatially separated; revealing a linear optically transparent slit 171C. Mask element 172 includes V-shaped optical elements 172A and 172B that block light collected in the pupil. Optical elements 172A and 172B are fixed relative to each other and spatially separated, revealing a V-shaped optically transparent slit 172C. Mask element 172 is movable across the pupil in the x-direction. In addition, mask element 172 is coupled to actuator 174. Actuator 174 is communicatively coupled to a computing system, e.g., computing system 140. In one example, computing system 140 communicates control commands 175 to actuator 174 that indicate a desired position of mask element 172 in the pupil. In response, actuator 174 translates mask element 172 to the desired position in the pupil.

[0075] As Figure 11 As depicted in FIG. 1, movement of mask element 172 in the x-direction changes the separation distance between aperture openings 173A and 173B in the pupil through which light is transmitted to detector 120. In Figure 11 In the embodiment depicted in FIG. 1, aperture openings 173A and 173B are symmetric around the x-axis and the distance between the aperture openings in the y-direction is determined by the x-position of mask element 172. In one embodiment, mask element 171 is positioned at the pupil plane 105 of inspection system 100 and mask element 172 is positioned at the pupil plane 106. Figure 1 As depicted in FIG. 1, movement of mask element 172 in the x-direction changes the separation distance between aperture openings 173A and 173B in the pupil through which light is transmitted to detector 120. In

[0076] Figure 12 In one embodiment, programmable pupil mask device 180 is depicted. As Figure 12 As depicted in FIG. 1, programmable pupil mask device 180 includes: mask element 181 that includes optical elements 181A-D that block light collected in the pupil; and mask element 182 that includes optical elements 182A-D that block light collected in the pupil. Optical elements 181A-D are fixed relative to each other, and optical elements 182A-D are fixed relative to each other. Optical elements 181A-D are spatially separated; revealing linear optically transparent slits aligned with the x-direction and the y-direction, respectively. Similarly, optical elements 182A-D are spatially separated; revealing linear optically transparent slits aligned with the x-direction and the y-direction, respectively.

[0077] The mask element 181 can be moved across the pupil in the x-direction. In addition, the mask element 181 is coupled to an actuator 184B. The actuator 184B is communicatively coupled to a computing system, e.g., the computing system 140. In one example, the computing system 140 communicates a control command 185B to the actuator 184B that indicates a desired position of the mask element 181 in the pupil. In response, the actuator 184B translates the mask element 181 to the desired position in the pupil. Similarly, the mask element 182 can be moved across the pupil in the y-direction. In addition, the mask element 182 is coupled to an actuator 184A. The actuator 184A is communicatively coupled to a computing system, e.g., the computing system 140. In one example, the computing system 140 communicates a control command 185A to the actuator 184A that indicates a desired position of the mask element 182 in the pupil. In response, the actuator 184A translates the mask element 182 to the desired position in the pupil.

[0078] As Figure 12 depicted in FIG. 1 IB, movement of the mask element 181 in the x-direction changes the position of the aperture opening 183B in the x-direction without moving the position of the aperture opening 183A. Similarly, movement of the mask element 182 in the y-direction changes the position of the aperture opening 183B in the y-direction without moving the position of the aperture opening 183A. In this way, phase difference measurements between several different positions in the pupil and a fixed point in the pupil are made by adjusting the position of the mask elements 181, 182 or both between each measurement. In one embodiment, the mask element 181 is positioned at the pupil plane 105 of the inspection system 100 depicted in FIG. 1 IB and the mask element 182 is positioned at the pupil plane 106. Figure 1 depicted in FIG. 1 IB and the mask element 182 is positioned at the pupil plane 106.

[0079] In some other embodiments, the programmable pupil mask device includes several different mask elements each having a fixed aperture pattern. The programmable pupil mask device includes an actuator subsystem (e.g., a linear translation stage, a rotational stage, etc.) to selectively position a desired mask element in a desired position in the pupil plane. In one example, the computing system 140 communicates a control command signal to the actuator subsystem. In response, the actuator subsystem positions the desired mask element in a desired position in the collection pupil plane of the optical system according to the control command signal.

[0080] In general, the computing system 140 is configured to detect features, defects, or light scattering properties of the wafer using the electrical signals obtained from each detector. The computing system 140 can include any suitable processor known in the art. In addition, the computing system 140 can be configured to use any suitable defect detection algorithm or method known in the art. For example, the computing system 140 can use die-to-database comparison or thresholding algorithms to detect defects on the sample.

[0081] Additionally, the inspection system 100 can include peripheral devices that can be used to accept input from an operator (e.g., a keyboard, a mouse, a touch screen, etc.) and display output to the operator (e.g., a display monitor). Input commands from the operator can be used by the computing system 140 to adjust the sampling locations within the collection pupil. The resulting sampling locations can be presented graphically to the operator on the display monitor.

[0082] The inspection system 100 includes a processor 141 and an amount of computer readable memory 142. The processor 141 and the memory 142 can be in communication through a bus 143. The memory 142 includes an amount of storage 144 that stores program code that, when executed by the processor 141, causes the processor 141 to perform the defect detection and classification functionality described herein.

[0083] Figure 13 A flowchart illustrating an exemplary method 200 that can be used to classify defects is shown. In some non-limiting examples, reference is made to the inspection system 100 described above. Figure 1 The inspection system 100 described is configured to implement the method 200. However, in general, implementation of the method 200 is not limited by the particular embodiments described herein.

[0084] In block 201, a first amount of illumination light is generated by an illumination source and directed to a measurement spot on a surface of a sample.

[0085] In block 202, a first amount of collected light is collected from the measurement spot on the surface of the sample in response to the first amount of illumination light. The first amount of collected light includes dark field scattered light within a collection pupil of a collection objective.

[0086] In block 203, a first portion of the first amount of collected light is blocked.

[0087] In block 204, a second portion of the first amount of collected light is transmitted. The second portion of the first amount of collected light is selected by one or more mask elements in a first configuration. The first amount of collected light is selected from at least two spatially distinct locations in the collection pupil.

[0088] In block 205, a first interference pattern formed by the second portion of the first amount of collected light is detected at or near a field plane that is conjugate to the surface of the sample.

[0089] In block 206, a first phase difference between transmitted light selected by the one or more mask elements in the first configuration from a first location of the at least two spatially distinct locations and transmitted light selected by the one or more mask elements in the first configuration from a second location of the at least two spatially distinct locations is determined from the first interference pattern.

[0090] Various embodiments of inspection systems or tools that can be used to inspect a sample are described herein. The term "sample" is used herein to refer to a wafer, reticle, or any other specimen that can be inspected for defects, characteristics, or other information (e.g., haze or film properties) as is known in the art.

[0091] As used herein, the term "wafer" generally refers to a substrate formed of semiconductor or non-semiconductor material. Examples include, but are not limited to, single crystalline silicon, gallium arsenide, and indium phosphide. Such substrates can typically be found and / or processed in a semiconductor fabrication facility. In some cases, a wafer can include only a substrate (i.e., a bare wafer). Alternatively, a wafer can include one or more layers of different materials formed on a substrate. The one or more layers formed on a wafer can be "patterned" or "unpatterned." For example, a wafer can include a plurality of dies having repeatable patterned features.

[0092] A "reticle" can be a reticle at any stage of a reticle fabrication process or a finished reticle that can or can not be released for use in a semiconductor fabrication facility. A reticle or "mask" is generally defined as a generally transparent substrate having generally opaque regions formed thereon and configured in a pattern. For example, the substrate can include a glass material, such as quartz. The reticle can be deposited over a photoresist-covered wafer during an exposure step of a photolithography process such that the pattern on the reticle can be transferred to the photoresist.

[0093] In one or more exemplary embodiments, the functions described can be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions can be stored on or transmitted over as one or more instructions or code on a computer-readable medium. Computer-readable media includes both computer storage media and communication media including any medium that facilitates transfer of a computer program from one place to another. A storage media can be any available media that can be accessed by a general purpose or special purpose computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code means in the form of instructions or data structures and that can be accessed by a general-purpose or special-purpose computer, or a general-purpose or special-purpose processor. Also, any connection is properly termed a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media.

[0094] Although some specific embodiments are described above for instructional purposes, the teachings of this patent document are not limited to those specific embodiments. In one example, the detector 120 can be replaced with an array of filters. In one example, the inspection system 100 can include more than one light source (not shown). The light sources can be configured in different or the same ways. For example, the light sources can be configured to generate light having different characteristics that can be directed at the same or different illumination zones at the same or different times at the same or different angles of incidence. The light sources can be configured according to any of the embodiments described herein. In addition, one of the light sources can be configured according to any of the embodiments described herein, and the other light source can be any other light source known in the art. In some embodiments, the inspection system can illuminate the wafer simultaneously over more than one illumination zone. The multiple illumination zones can be spatially overlapping. The multiple illumination zones can be spatially distinct. In some embodiments, the inspection system can illuminate the wafer over more than one illumination zone at different times. The different illumination zones can be temporally overlapping (i.e., illuminated simultaneously for some period of time). The different illumination zones can be temporally distinct. In general, the number of illumination zones can be arbitrary, and each illumination zone can have equal or different sizes, orientations, and angles of incidence. In yet another example, the inspection system 100 can be a scanning spot system with one or more illumination zones that are scanned independently of any motion of the wafer 110. In some embodiments, the illumination zones are scanned in a repeating pattern along a scan line. The scan line can or can not be aligned with a scan motion of the wafer 110. Although as presented herein, the wafer positioning system 125 generates motion of the wafer 110 through coordinated rotational and translational movements, in yet another example, the wafer positioning system 125 can generate motion of the wafer 110 through coordinated two translational movements. For example, the wafer positioning system 125 can generate motion along two orthogonal linear axes (e.g., X-Y motion). In such embodiments, a scan pitch can be defined as the distance between adjacent translational scans along either motion axis. In such embodiments, the inspection system includes an illumination source and a wafer positioning system. The illumination source supplies an amount of radiation to a surface of the wafer over an illumination zone. The wafer positioning system moves the wafer in a scan motion characterized by a scan pitch (e.g., scans back and forth in one direction and steps by an amount equal to the scan pitch in an orthogonal direction).

[0095] Thus, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced with modifications and variations of the described embodiments using materials and concepts similarly partici pating in the spirit of the invention as recited in the claims.

Claims

1. A system for surface inspection, comprising: an illumination source configured to generate a first amount of illumination light directed to a defect of interest disposed on a sample and a second amount of illumination light; a collection objective configured to collect a first amount of collected light from the defect of interest in response to the first amount of illumination light and a second amount of collected light from the defect of interest in response to the second amount of illumination light, the first and second amounts of collected light including dark field scattered light within a collection pupil of the collection objective; one or more mask elements disposed in a first configuration or disposed in a second configuration, the one or more mask elements in the first configuration blocking a first portion of the first amount of collected light and transmitting a second portion of the first amount of collected light, the second portion of the first amount of collected light selected from at least two spatially distinct regions in the collection pupil, the one or more mask elements in the second configuration blocking a first portion of the second amount of collected light and transmitting a second portion of the second amount of collected light, the second portion of the second amount of collected light selected from at least two spatially distinct regions in the collection pupil, wherein at least one of the at least two spatially distinct regions in the second configuration is different than at least one of the at least two spatially distinct regions in the first configuration; an imaging detector having a photosensitive surface located at or near a field plane conjugate to a surface of the sample, the imaging detector configured to detect a first interference pattern formed by the transmitted portion of the first amount of collected light at or near the field plane and to detect a second interference pattern formed by the transmitted portion of the second amount of collected light at or near the field plane; and one or more processors configured to: receive output signals indicative of the first and second interference patterns; obtain a first phase difference between transmitted light from a first region of the at least two spatially distinct regions in the first configuration and transmitted light from a second region of the at least two spatially distinct regions in the first configuration based on the first interference pattern and a second phase difference between transmitted light from a first region of the at least two spatially distinct regions in the second configuration and transmitted light from a second region of the at least two spatially distinct regions in the second configuration based on the second interference pattern; and classify the defect of interest based on the first and second phase differences.

2. The system of claim 1, wherein the classification of the defect involves determining a material composition of the defect based on the first and second phase differences.

3. The system of claim 1, wherein the one or more mask elements are located at or near a pupil plane of the collection objective, a pupil plane conjugate to the pupil plane of the collection objective, or a combination thereof.

4. The system of claim 1, wherein a spatial separation between the first region of the at least two spatially distinct regions in the collection pupil and the second region of the at least two spatially distinct regions in the collection pupil spans a range from 0.1 numerical aperture to 0.9 numerical aperture.

5. The system of claim 1, wherein the first region of the at least two spatially distinct regions in the collection pupil and the second region of the at least two spatially distinct regions in the collection pupil are symmetrically located about a center of the collection pupil.

6. The system of claim 1, wherein a size of a first aperture opening formed by the one or more mask elements in the first configuration at the first region of the at least two spatially distinct regions in the collection pupil is in a range from 0.01 numerical aperture to 0.3 numerical aperture, and a size of a second aperture opening formed by the one or more mask elements in the first configuration at the second region of the at least two spatially distinct regions in the collection pupil is in a range from 0.01 numerical aperture to 0.3 numerical aperture.

7. The system of claim 6, wherein the size of the first aperture opening is different than the size of the second aperture opening.

8. The system of claim 6, further comprising: a neutral density filter disposed in an optical path of the transmitted light selected by the one or more mask elements in the first configuration at or near the first region where the first aperture opening is formed by the one or more mask elements.

9. The system of claim 1, further comprising: a first actuator coupled to a first mask element of the one or more mask elements, the first actuator communicatively coupled to the one or more processors, wherein the first actuator moves the first mask element to a first desired position in response to a first control command communicated from the one or more processors to the first actuator.

10. The system of claim 9, further comprising: a second actuator coupled to a second mask element of the one or more mask elements, the second actuator communicatively coupled to the one or more processors, wherein the second actuator moves the second mask element to a second desired position in response to a second control command communicated from the one or more processors to the second actuator.

11. The system of claim 1, the collection objective having a numerical aperture spanning at least a range from 0.1 numerical aperture to 0.99 numerical aperture.

12. A method for surface inspection, comprising: generating a first amount of illumination light and a second amount of illumination light directed to a defect of interest disposed on a sample; collecting a first amount of collected light from the sample in response to the first amount of illumination light; collecting a second amount of collected light from the sample in response to the second amount of illumination light, the first and second amounts of collected light including dark-field scattered light within a collection pupil of a collection objective; blocking a first portion of the first amount of collected light; transmitting a second portion of the first amount of collected light, the second portion of the first amount of collected light selected from at least two spatially distinct regions in the collection pupil by one or more mask elements in a first configuration; blocking a first portion of the second amount of collected light; transmitting a second portion of the second amount of collected light, the second portion of the second amount of collected light selected from at least two spatially distinct regions in the collection pupil by one or more mask elements in a second configuration, wherein at least one of the at least two spatially distinct regions in the second configuration is different than at least one of the at least two spatially distinct regions in the first configuration; detecting a first interference pattern formed by the transmitted portion of the first amount of collected light at or near a field plane conjugate to a surface of the sample; detecting a second interference pattern formed by the transmitted portion of the second amount of collected light at or near the field plane conjugate to the surface of the sample; obtaining a first phase difference between transmitted light from a first region of the at least two spatially distinct regions in the first configuration and transmitted light from a second region of the at least two spatially distinct regions in the first configuration based on the first interference pattern and a second phase difference between transmitted light from a first region of the at least two spatially distinct regions in the second configuration and transmitted light from a second region of the at least two spatially distinct regions in the second configuration based on the second interference pattern; and classifying the defect of interest based on the first and second phase differences.

13. The method of claim 12, wherein the one or more mask elements are located at or near a pupil plane of the collection objective, a pupil plane conjugate to the pupil plane of the collection objective, or a combination thereof.

14. The method of claim 12, wherein the classifying of the defect involves determining a material composition of the defect based on the first and second phase differences.

15. A system for surface inspection, comprising: an illumination source configured to generate a first amount of illumination light and a second amount of illumination light directed to a defect of interest disposed on a sample; a collection objective configured to collect a first amount of collected light from the defect of interest in response to the first amount of illumination light and a second amount of collected light from the defect of interest in response to the second amount of illumination light, the first and second amounts of collected light including dark-field scattered light within a collection pupil of the collection objective; ​ one or more mask elements disposed in a first configuration or disposed in a second configuration, the one or more mask elements in the first configuration blocking a first portion of the first amount of collected light and transmitting a second portion of the first amount of collected light, the second portion of the first amount of collected light selected from at least two spatially distinct regions in the collection pupil, the one or more mask elements in the second configuration blocking a first portion of the second amount of collected light and transmitting a second portion of the second amount of collected light, the second portion of the second amount of collected light selected from at least two spatially distinct regions in the collection pupil, wherein at least one of the at least two spatially distinct regions in the second configuration is different than at least one of the at least two spatially distinct regions in the first configuration; an imaging detector having a light sensitive surface located at or near a field plane conjugate to a surface of the sample, the imaging detector configured to detect a first interference pattern formed by the transmitted portion of the first amount of collected light at or near the field plane and to detect a second interference pattern formed by the transmitted portion of the second amount of collected light at or near the field plane; and a non-transitory computer readable medium storing instructions that, when executed by one or more processors, cause the one or more processors to: receive output signals indicative of the first interference pattern and the second interference pattern; obtain a first phase difference between transmitted light from a first region of the at least two spatially distinct regions in the first configuration and transmitted light from a second region of the at least two spatially distinct regions in the first configuration based on the first interference pattern and a second phase difference between transmitted light from a first region of the at least two spatially distinct regions in the second configuration and transmitted light from a second region of the at least two spatially distinct regions in the second configuration based on the second interference pattern; and classify the defect of interest based on the first phase difference and the second phase difference.

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