Patent Publication Number: US-2023152241-A1

Title: Simultaneous back and/or front and/or bulk defect detection

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 USC 119 to U.S. Provisional Application No. 63/280,949, entitled “SIMULTANEOUS BACK AND/OR FRONT AND/OR BULK DEFECT DETECTION,” filed Nov. 18, 2021, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to inspection systems, in particular high throughput substrate inspection systems with simultaneous back, front, and/or bulk defect detection with submicron sensitivity. 
     BACKGROUND 
     Semiconductor and other similar industries, often use optical metrology equipment to provide non-contact evaluation of substrates during processing. Optical metrology is often used to determine one or more characteristics of a sample or features on the sample. Another type of evaluation of samples is defect inspection. Defects, such as particles or other irregularities on a sample, may interfere with the performance of resulting devices. Conventionally, optical tools used to detect defects use bright-field and dark-field inspection. Bright-field and dark-field detection tools detect defects based on the scattering of light caused by defects. Improvements of optical tools used for defect inspection are desired. 
     SUMMARY 
     An inspection system for inspecting multiple surfaces of a substrate includes at least one illuminator that produces light at a first wavelength directed at a substrate at a first angle light at a second wavelength directed at the substrate at an oblique angle. An adjustment system may be present to adjust the oblique angle. The substrate may be opaque to one of the wavelengths and at least partially transparent to the other wavelength. Detection optics collect backscattered light from a first surface and a second surface of the substrate in response to the light. At least one detector receives the backscattered light and generates a first image representative of the first surface of the substrate and a second image representative of a second surface or near the second surface of the substrate. The images may be compared to generate a third image representative of defects on or near the second surface of the substrate corrected for residual signals of defects on the first surface. 
     In one implementation, an inspection system for substrate inspection includes at least one illuminator that generates light at a first wavelength directed at a substrate at a first angle and generates light at a second wavelength directed at the substrate at a second angle that is an oblique angle with respect to a first surface of the substrate. An adjustment system in the inspection system adjusts the second angle to focus the light at the second wavelength at a specified location. The inspection system includes detection optics that collect backscattered light from the substrate in response to the light from the at least one illuminator. The inspection system further includes at least one detector that receives the backscattered light in a first spectrum and generates a first image representative of the first surface of the substrate, and receives the backscattered light in a second spectrum to generate a second image representative of a second surface or near the second surface of the substrate. 
     In one implementation, a method for inspecting a substrate includes generating light at a first wavelength directed at a substrate at a first angle and generating light at a second wavelength directed at the substrate at a second angle that is an oblique angle with respect to a first surface of the substrate. The second angle may be adjusted to focus the light at the second wavelength at a specified location. Backscattered light is collected from the substrate in response to the light at the first wavelength and the second wavelength. The method includes generating a first image representative of the first surface of the substrate based on the backscattered light in a first spectrum and generating a second image representative of a second surface or near the second surface of the substrate based on the backscattered light in a second spectrum. 
     In one implementation, an inspection system includes a means for generating light at a first wavelength directed at a substrate at a first angle and a means for generating light at a second wavelength directed at the substrate at a second angle that is an oblique angle with respect to a first surface of the substrate. The inspection system includes a means for adjusting the second angle to focus the light at the second wavelength at a specified location. A means for collecting collects backscattered light from the substrate in response to the light at the first wavelength and the second wavelength. The inspection system further includes a means for generating a first image representative of the first surface of the substrate based on the backscattered light in a first spectrum and a means for generating a second image representative of a second surface or near the second surface of the substrate based on the backscattered light in a second spectrum. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A and  1 B  illustrate example of an inspection system in a front surface inspection configuration and a back surface inspection configuration, respectively. 
         FIG.  2    illustrates portions of an inspection system that includes two channels for simultaneous inspection of multiple surfaces of a substrate using two different wavelengths of light. 
         FIGS.  3 A and  3 B  respectively illustrate an image (defect map) of a first surface of a substrate produced in response to light having a first wavelength and an image (defect map) of a second surface of a substrate produced in response to light having a second wavelength. 
         FIG.  3 C  illustrates a comparison of the images (defect maps) shown in  FIGS.  3 A and  3 B  to reduce or eliminate residual signals from the defects from the first surface in the image (defect map) of the second surface. 
         FIG.  4    illustrates a close view of a portion of the detection optics of the inspection system. 
         FIG.  5    illustrates a close view of a portion of the detection optics of the inspection system. 
         FIG.  6    is a flow chart illustrating a method for inspecting a substrate. 
     
    
    
     DETAILED DESCRIPTION 
     Optical systems may be employed for various inspection applications. For example, semiconductor and other similar industries, often use optical metrology equipment to provide non-contact evaluation of substrates during processing. Samples, such as semiconductor wafers, may be inspected during processing to detect defects on the sample, e.g., by illuminating the sample and detecting backscattered light from the sample. One important performance criteria for inspection systems is throughput, as the samples are typically inspected during processing. Inspection systems, however, are typically limited with respect to throughput. For example, inspection systems typically inspect either a front side or back side of a sample at one time. Thus, inspection of more than one surface increases the inspection time, thereby reducing throughput and minimizing contact with the substrate, thereby improving cleanliness of the substrates. 
     As discussed herein, improvements to an inspection system and design are provided by enabling simultaneously inspection of two or more of the backside of the sample, frontside of the sample, and bulk characteristics of the sample (e.g., semiconductor wafer) while maintaining high throughput. The inspection system, for example, may include two channels: a first channel may use light with a first wavelength that is directed at a substrate at a first angle to detect defects on the first surface of a substrate, and a second channel may use light with a second wavelength that is directed at the substrate at a second angle that is an oblique angle to detect defects on or near a second surface of the substrate. An adjustment system may be present to adjust the second angle to focus the light with the second wavelength at a specified location. The first wavelength may be in the visible spectrum to detect defects on the first surface, such as the proximal/near surface of the object (e.g., backside of a wafer), and the second wavelength may be in the infrared or near-infrared spectrum to detect defects on or near the distal/opposing surface of the object (frontside of a wafer) and/or bulk characteristics. The light with the first wavelength may be directed at or near a normal angle with respect to the substrate. 
     Techniques described herein add a secondary, different wavelength set of illumination and detector which may be used for dual purpose defect inspection. By setting the primary wavelength (e.g., in the visible spectrum) so that the substrate of interest is opaque, while setting the secondary wavelength (e.g., in the infrared or near infrared spectrum) so that the substrate of interest is at least partially transparent, the primary wavelength may be used for defect inspection on the near surface while the secondary wavelength may be used for defect inspection on the opposing surface or for measuring bulk characteristics. In addition, comparison of both the primary and secondary channel can determine whether detected defects are located on the near surface (in which case both channels will receive the signal) or in the bulk or opposing surface (in which case only the secondary channel will receive the signal). 
       FIG.  1 A  illustrates example portions of an inspection system  100  in a conventional front surface inspection configuration. The inspection system  100  includes an inspection head  101 , e.g., including an illumination device and detector. The inspection head  101  illuminates the front surface  104  of a wafer  102  and detects backscattered light from defects on the front surface  104 . Because the inspection head  101  illuminates and detects backscattered light from the near surface of the wafer, the inspection head  101  may use wavelengths (e.g., in the visible spectrum) that do not penetrate the wafer  102 , i.e., the wafer  102  is opaque to the wavelengths used by the inspection head  101 . The inspection system  100  may include a rotating wafer chuck (not shown) for holding the wafer  102  and rotating the wafer  102  during inspection (as illustrated by curved arrow  106 ). For example, the chuck may be provided as a backside vacuum chuck. The inspection head  101  may additionally scan across the wafer  102 , e.g., from the center radially outward (as illustrated by arrow  108 ). The inspection head  101 , for example, may be on an arm or a track. 
       FIG.  1 B  illustrates example portions of an inspection system  150  in a back surface inspection configuration. The inspection system  150  includes an inspection head  151  (similar to inspection head  101  shown in  FIG.  1 A ), e.g., including an illumination device and detector. The inspection head  151  illuminates the back surface  154  of a wafer  152  and detects backscattered light from defects on the back surface  154 . Similar to inspection head  101 , because the inspection head  151  illuminates and detects backscattered light from the near surface of the wafer, the inspection head  151  may use wavelengths (e.g., in the visible spectrum) that do not penetrate the wafer  152 , i.e., the wafer  152  is opaque to the wavelengths used by the inspection head  151 . The inspection system  150  may include a rotating wafer chuck  155  for holding the wafer  152  and rotating it during inspection (as illustrated by curved arrow  156 ). For example, the chuck  155  may be provided as an edge clamp, which may provide clearance behind the wafer  152 . The inspection head  151  may scan across the wafer  152 , e.g., from the center radially outward (as illustrated by arrow  158 ). 
     As discussed above, with the use of conventional inspection systems, such as illustrated in  FIGS.  1 A and  1 B , only one surface of the substrate is inspected at a time. Consequently, if both the top surface and bottom surface are to be inspected, one surface of the substrate is inspected by one inspection system before transferring the substrate to a second inspection system where the other surface may be inspected. Consequently, the inspection time is increased, which reduces throughput and increases wafer contacts which increases the risk of contamination. 
       FIG.  2    illustrates example portions of a cross-section of an inspection system  200  that includes two channels for simultaneous inspection of multiple surfaces of a substrate  202  using two different wavelengths of light. The inspection system  200  is illustrated in a back surface inspection configuration with an edge clamp chuck  205 , but in some implementations, the inspection system  200  may be provided in a front surface inspection configuration, e.g., with a backside vacuum chuck. 
     The inspection system  200  includes at least one illuminator that generates light different wavelengths that are incident on the substrate  202  at different angles of incidence. For example, as illustrated, the inspection system  200  includes a first channel with a first light source  210  that generates light  212 , which has a first wavelength, that is incident on the bottom surface  204  of the substrate  202  at a normal or near normal angle of incidence, and a second channel with a second light source  220  that generates light  222 , which has a second wavelength, that is incident on the bottom surface  204  of the substrate  202  at an oblique angle of incidence. If desired, a single light source may be used that generates light with different wavelengths, where light with a first wavelength may be directed along the first channel and light with the second wavelength may be directed along the second channel, e.g., using a chromatic beam splitter. 
     The first light source  210  in the first channel, for example, may be a laser or other appropriate high brightness light source. The first light source  210  operates at the first wavelength, which may be in the visible spectrum. The first wavelength may be chosen based on the surface properties of the material of the substrate  202  being inspected. For example, the first light source  210  may produce light between, e.g., 400-700 nm for silicon wafers. The first light source  210  and optical elements, such as turning mirror  211  and turning mirror  234  of detection optics  230  and focusing optics (not shown), may be configured so that the light  212  produced by the first light source  210  is at normal angle of incidence or near normal angle of incidence to the substrate  202  being inspected. In some implementations, the first light source  210  and optical elements may be mounted so that light  212  is slightly off normal (e.g., 0-10° offset to normal). The first wavelength generated by the first light source  210  is selected so that the substrate  202  is opaque to light  212  and, thus, the first light source  210  is used to detect defects on the proximal surface  204  of the substrate  202 , which in the example shown in  FIG.  2    is the back surface of the substrate  202 . 
     The second light source  220  in the second channel, for example, may be a laser or other appropriate high brightness light source. The second light source  220  operates at the second wavelength, which may be in the infrared or near-infrared spectrum. The second wavelength may be chosen based on the transmittance properties of the material of the substrate  202  being inspected. For example, the second light source  220  may produce light between, e.g., 700-1400 nm for inspecting silicon wafers. The second light source  220  and any optical elements, such as focusing optics (not shown) may be mounted so that the light  222  produced by the second light source  220  is incident at an oblique angle of incidence to the substrate  202 . The second wavelength generated by the second light source  220  is selected so that the substrate  202  is at least partially transparent to the light  222  and, thus, the second light source  220  is used to detect defects on or near the distal/opposing surface  206  of the substrate  202 , which in the example shown in  FIG.  2    is the front surface of the substrate  202 , or to detect defects in bulk between the proximal surface  204  and the distal/opposing surface  206 . The second wavelength, e.g., in the infrared or near infrared spectrum, penetrates the substrate  202  and, thus, can be used to detect defects in the volume of material and/or on or near the distal/opposing surface  206 . Accordingly, the first light source  210  and the second light source  220  may operate at the same time and at the same location of the substrate  202  to detect defects on the front surface, back surface, and the bulk of the substrate  202 . 
     It should be understood that while the first light source  210  is described herein as producing normal or near normal light, in some implementations, the light source  210  may produce light  212  that is incident on the substrate  202  at an oblique angle. Moreover, in some implementations, the second light source  220  may be mounted to produce light  222  that is normal or near normal, which may assist in detecting bulk defects. Further, in some implementations, the first light source  210  and the second light source  220  may be mounted on different sides of the substrate  202 . For example, the first light source  210  may be mounted on the back side of the substrate  202 , as illustrated, to perform surface inspection of the back surface in the visible spectrum, while the second light source  220  may be mounted on the top side of the substrate  202  to perform surface and bulk inspection in the infrared spectrum. 
     The inspection system  200  further includes detection optics  230  that collect backscattered light from the substrate  202 . Both the first channel and second channel may use the same detection optics  230 , which may have limited focus in X, Y, Z directions as discussed below in reference to  FIG.  4   . As illustrated, the detection optics  230  may include an elliptical mirror  232  and one or more turning mirrors  234 . The elliptical mirror  232  may collect the backscattered light from both channels, e.g., backscattered light  214  produced by the proximal surface  204  of the substrate  202  in response to light  212  and backscattered light  224  produced by the distal/opposing surface  206  or bulk material of the substrate  202  (and may include some backscattered light from the proximal surface  204  of the substrate  202 ) in response to light  222 . The turning mirror  234  may redirect the collected backscattered light to one or more detectors. As illustrated, the turning mirror  234  of the detection optics  230  includes an aperture  235  through which the light  212  may pass to be normally incident on the substrate  202 . In some implementations, the elliptical mirror  232  of the detection optics  230  may include two slits, one slit provided for the light  222  to pass through to the substrate  202  and a second slit provided for light  222  reflected from the substrate  202  to pass through. 
     It should be understood that examples discussed herein sometimes refers to the use of darkfield imaging where collected backscattered light is representative of defects, but in some implementations, brightfield imaging may also or alternatively be used where collected reflected light of reduced intensity may be representative of defects. 
     The inspection system  200  further includes at least one detector that receives the backscattered light and generates images representative of the surfaces of the substrate  202 . For example, as illustrated, the inspection system  200  includes a chromatic filter  240  that receives the backscattered light  214  and  224  from the turning mirror  234  after passing through aperture  242  and separates the backscattered light  214  and  224  based on wavelength. The backscattered light  214 , which is produced in response to incident light  212 , may be in a first spectrum that includes one or more visible wavelengths and the backscattered light  224 , which is produced in response to incident light  222 , may be in a second spectrum that includes one or more infrared or near infrared wavelengths. The chromatic filter  240 , for example, may be a dichroic filter that directs the backscattered light  214  in response to the light  212  from the first light source  210  in the first channel to a first detector  216 , and directs backscattered light  224  in response to the light  222  from the second light source  220  in the second channel to a second detector  226 . Thus, one aperture  242  may be used for both detectors  216  and  226 . In some implementations, the chromatic filter  240  may be located before the focal point for each detector  216 ,  226 , and separate apertures may be used for each detector  216 ,  226 , as opposed to the single aperture  242  shown in  FIG.  2   . Use of independent apertures, for example, may be useful as the backscattered light  214  and  224  originate in different z locations of the substrate and accordingly have different focal points. The use of a single aperture  242  for both detectors  216 ,  226 , however, may be desirable if there is marginal impact as it simplifies the architecture, reduces space and improves robustness. 
     The chromatic filter  240  may separate the backscattered light into different spectrums. For example, chromatic filter  240  may allow visible light to pass through to the first detector  216  and reflect infrared light to the second detector  226 . In some implementations, the chromatic filter  240  may reflect visible light and may allow infrared light to pass through. In other implementations, the chromatic filter  240  may be replaced with a diffraction element that may separate the wavelengths of the backscattered light, which are directed to and detected by different portions of a single two dimensional detector. 
     In some examples, apertures may be used to focus the backscattered light. For example, apertures may further limit the X, Y focus of the elliptical mirror. The smaller the aperture, the more limited the X, Y focus. Apertures may be placed before or after the chromatic filter  240  to increase the X, Y focus. 
     The first detector  216  may be configured to detect light, e.g., in the visible spectrum. For example, the first detector  216  may include avalanche photodiodes or photomultipliers. The second detector  226  may be configured to detect light in the infrared spectrum. For example, the second detector  226  may include avalanche photodiodes suited for infrared operations, such as an indium gallium arsenide (InGaAs) photodiode that is sensitive to infrared. 
     The first detector  216  (in combination with a processor) may generate images showing defects on the proximal (back) surface  204  of the substrate  202 . The second detector  226  (in combination with the processor) may generate images showing defects on or near the distal (front) surface  206  of the substrate  202 , or in bulk. For X, Y positions of the substrate  202  inspected by the inspection system  200 , the inspection system  200  generates two separate signals, one for the backscattered light  214  produced in response to the first light  212  and the second for the backscattered light  224  produced in response to the second light  222 . 
     In addition, in some implementations, the inspection system  200  may include an adjustment system  250  that adjusts the angle of the light  222  from the second light source  220  to focus the light  222  at a specified location of the substrate  202 . As illustrated, the light  222  from the second light source  220  may be redirected at an oblique angle of incidence using the adjustment system  250 . The adjustment system  250  may be used to improve inspection performance (e.g., sensitivity, repeatability,) and ease physical alignment constraints during build and calibration of the inspection system  200 . The adjustment system  250 , for example, may be used during calibration to properly align the area of incident of light  222 , e.g., with the area of incidence of light  212 . The adjustment system  250  may operate dynamically during substrate  202  inspection to equalize any substrate deformations or misalignment which otherwise may reduce signal repeatability. The adjustment system  250 , for example, may include one or more moving minors. As illustrated, a first mirror  252  (e.g., a 45° mirror) may redirect the light  222  to a second mirror  254 . The second mirror  254  may be coupled to an actuator  256  (e.g., piezoelectric), which may be controlled by a computing system, such as computing system  260 , or another controller, such as a FPGA or logic embedded in an alignment sensor  258 . The orientation of the second mirror  254  may be adjusted automatically during runtime to keep the second light source  220  focused on the specified spot in the substrate  202 , e.g., at the distal surface  206 , which may vary in the substrate  202  along the z axis. Some substrates  202 , for example, may be deformed so that the second light source  220  may be adjusted during inspection to keep the focus at the specified z axis location of the substrate  202 . The adjustment may be done based on the output of the alignment sensor  258  that detects the light  222  reflected from the substrate  202 . This alignment sensor  258 , for example, may be provided as a one dimensional sensor and may measure the position of the reflected light. In another implementation, a camera system may monitor the region of interest (from normal incidence or at oblique incidence) or to measure the position at which the light  222  is incident on the substrate. Based on the measured position of the reflected light, the second mirror  254  may be adjusted to focus the light  222  on the specified location of the substrate  202 . Moreover, the elliptical mirror of the detection optics may include two slits, one slit provided for the light  222  to pass through to the substrate  202  and the second slit provided for the reflected light to exit the elliptical mirror  232  and to reach the alignment sensor  258 . 
     Inspection system  200  further includes one or more computing systems  260  that is configured to perform inspection of the substrate  202  as described herein. The one or more computing systems  260  is coupled to the first detector  216  and the second detector  226  to receive the inspection data acquired by the detectors  216 ,  226  during inspection of the substrate  202 . The one or more computing systems  260 , for example, may be a workstation, a personal computer, central processing unit or other adequate computer system, or multiple systems. The one or more computing systems  260  may be configured to control the inspection process as well as to analyze the inspection data, e.g., in accordance with the methods described herein. 
     It should be understood that the one or more computing systems  260  may be a single computer system or multiple separate or linked computer systems, which may be interchangeably referred to herein as computing system  260 , at least one computing system  260 , one or more computing systems  260 . The computing system  260  may be included in or is connected to or otherwise associated with inspection system  200 . Different subsystems of the inspection system  200  may each include a computing system that is configured for carrying out steps associated with the associated subsystem. The computing system  260 , for example, may control the positioning of the substrate  202 , e.g., by controlling movement of a stage  207  coupled to the chuck  205 , by controlling movement of the inspection head, e.g., the optical elements of the inspection system  200 , or by controlling movement of both the stage  207  and the inspection head, e.g., via one or more actuators. The stage  207  and/or inspection head, for example, may be capable of horizontal motion in either Cartesian (i.e., X and Y) coordinates, or Polar (i.e., R and θ) coordinates or some combination of the two. In some implementations, for example, the stage  207  is moved rotationally (R), while the inspection head is moved laterally (θ). The stage  207  and/or the inspection head, e.g., optical components of the inspection system  200 , may also be capable of vertical motion along the Z coordinate. Vertical motion of the inspection head along the Z coordinate may be particularly useful for adoption to different substrate types or thicknesses in a front side inspection with a backside vacuum chuck. The computing system  260  may further control the operation of the chuck  205  to hold or release the substrate  202 . 
     The computing system  260  may be communicatively coupled to the first detector  216  and the second detector  226  in any manner known in the art. For example, the one or more computing systems  260  may be coupled to separate computing systems that are associated with the detectors  216 ,  226 . The computing system  260  may be configured to receive and/or acquire inspection data or information from one or more subsystems of the inspection system  200 , e.g., the detectors  216 ,  226 . The transmission medium, thus, may serve as a data link between the computing system  260  and other subsystems of the inspection system  200 . 
     The computing system  260 , which includes at least one processor  262  with memory  264 , as well as a user interface (UI)  268 , which are communicatively coupled via a bus  261 . The memory  264  or other non-transitory computer-usable storage medium, includes computer-readable program code  266  embodied thereof and may be used by the computing system  260  for causing the at least one computing system  260  to control the inspection system  200  and to perform the functions including the analysis described herein, including adjusting the angle of the second light  222  and generating and analyzing images of multiple surfaces of a substrate based on the inspection data provided by the detectors  216  and  226  to detect defects and to report detected defects, as discussed herein. The data structures and software code for automatically implementing one or more acts described in this detailed description can be implemented by one of ordinary skill in the art in light of the present disclosure and stored, e.g., on a computer-usable storage medium, e.g., memory  264 , which may be any device or medium that can store code and/or data for use by a computer system, such as the computing system  260 . The computer-usable storage medium may be, but is not limited to, include read-only memory, a random access memory, magnetic and optical storage devices such as disk drives, magnetic tape, etc. Additionally, the functions described herein may be embodied in whole or in part within the circuitry of an application specific integrated circuit (ASIC) or a programmable logic device (PLD), and the functions may be embodied in a computer understandable descriptor language which may be used to create an ASIC or PLD that operates as herein described. 
     The results from the analysis of the data may be reported, e.g., stored in memory  264  associated with the substrate  202  and/or indicated to a user via UI  268 , an alarm or other output device. Moreover, the results from the analysis may be reported and fed forward or back to the process equipment to adjust the appropriate fabrication steps to compensate for any detected variances in the fabrication process. The computing system  260 , for example, may include a communication port  269  that may be any type of communication connection, such as to the internet or any other computer network. The communication port  269  may be used to receive instructions that are used to program the computing system  260  to perform any one or more of the functions described herein and/or to export signals, e.g., with inspection results and/or instructions, to another system, such as external process tools, in a feed forward or feedback process. 
       FIGS.  3 A and  3 B , by way of example respectively illustrate an image  300  (sometimes referred to as a defect map  300 ) generated based on the output of the first detector  216  and an image  310  (sometimes referred to as a defect map  310 ) generated based on the output of the second detector  226 . The image  300  shown in  FIG.  3 A  shows examples of defects  302 ,  304 ,  306 , and  308  on the proximal surface detected using light  212  from the first light source  210 . Th image  310  shown in  FIG.  3 B  shows defects  312 ,  314 , and  316  on or near the distal surface using light  222  from the second light source  220 , as well as some residual signal from the defects  306  and  308  of the proximal-side signal. 
     Hence, the two images  300  and  310  may be compared, e.g., by the computing system  260 , and any residual proximal side signals in the image of the distal surface can be eliminated. For example, the two images  300  and  310  generated based on inspection data from the first detector  216  and the second detector  226  may be overlaid on top of each other. The position of the images  300  and  310  should be aligned because the same detection optics  230  are used. The defects found in the proximal surface (backside in the example illustrated in  FIG.  2   ) may be subtracted from the distal surface (frontside in the example illustrated in  FIG.  2   ). 
       FIG.  3 C , by way of example, illustrates a modified image  320  of the distal surface. Image  320  only shows defects  312 ,  314 , and  316  on the distal surface (here, front surface) because the residual signals from the defects  306  and  308  in the proximal surface (back surface) may be reduced or eliminated based on the comparison of the two images  300  and  310 . 
       FIG.  4    illustrates a close view of a portion of the inspection system  200  including the second light source  220  and elliptical mirror  232  of the detection optics  230  and the substrate  202 .  FIG.  4    shows the obliquely incident light  222  produced by the second light source  220  and resulting backscattered light  224  from the distal surface  206  of the substrate  202 , as well as the normally (or near normal) incident light  212  produced by the first light source (not shown in  FIG.  4   ) and the resulting backscattered light  214  from the proximal surface  204  of the substrate  202 . As illustrated, the light  222  from the second light source  220  may be incident on the proximal surface  204  at outside the x/y focus of detection optics  230  (illustrated by elliptical mirror  232 ) thereby producing reducing defect scattered intensities by the light  222  except at the distal/opposing surface  206 . In other words, due to the lateral X, Y offset of the light  222  with respect to the detection optics  230 , the backscattered light responsive to the light  222  from the second light source  220  are collected by the detection optics  230  for signals coming from or near the distal surface  206 . 
     Also, the light  222  generated by the second light source  220  that is incident on the proximal surface  204  of the substrate  202  may be outside its own z focus, further reducing defect scattered intensities except at the distal/opposing surface  206 . Thus, the second channel has increased signal sensitivity at and around the distal/opposing surface  206 . The bulk and defects on the proximal surface  204  have significantly reduced signal sensitivity in the second channel. Thus, the use of the oblique angle of incidence of light  222  from the second light source  220  allows simultaneous front and back surface inspection. Moreover, by reducing the incident angle of the second light source  220  (i.e., so that light  222  is incident at a more oblique angle), the sensitivity increases throughout the substrate thickness. Defects in bulk may generate a strong signal and can be detected with increased sensitivity. 
     Due to residual absorption within the material of the substrate  202 , defect signals decrease the farther the light  222  travels through the substrate  202 . On the other hand, the incident angle and focus alignment of the second light source  220  in conjunction with the x/y/z focus of the detection optics  230 , e.g., elliptical mirror  232 , cause the opposite effect, i.e., the closer a defect to the distal surface  206 , the stronger the defect signal. Accordingly, deliberate alignment of the incident angle of the light  222  from the second light source  220  may be used so that both effects can compensate each other resulting in same strength signals of same defect sizes along the z axis through the substrate  202 . Signal strength is correlated to defect size, and accordingly, the size accuracy for the inspection system  200  may become independent of z position of the defect. 
       FIG.  5    illustrates a portion of an inspection system  500 , which may be similar to inspection system  200  shown in  FIG.  2   , but includes detection optics  530  with apertures through which the oblique light  522  passes, and is illustrated in a front surface inspection configuration with a backside vacuum chuck  505  to hold the substrate  502 . The chromatic filter and detectors are not illustrated in  FIG.  5    for the sake of simplicity. 
       FIG.  5    illustrates normal (or near normal) incident light  512  from a first light source  510  that passes through an aperture  535  of a turning mirror  534  of the detection optics  530 .  FIG.  5    further illustrates oblique incident light  522  from a second light source  520 .  FIG.  5    does not illustrate an adjustment system for the oblique incident light  522 , but an adjustment system may be present if desired. As illustrated, the elliptical mirror  532  of the detection optics  530 , which collects backscattered light  514  and  524 , includes a first aperture  532   a , through which the light  522  passes and is obliquely incident on the substrate  502 , and a second aperture  532   b , through which the light  522  reflected from the substrate  502  exits the elliptical mirror  532 . If desired, the apertures  532   a  and  532   b  may be extended, e.g., slits, to accommodate changes in the angle of incidence. Moreover, the aperture  532   b  may be extended so that light  522  reflected from both the proximal (top) surface  504  and the distal (bottom) surface  506  of the substrate  502  may pass and exit the elliptical mirror  532 . 
       FIG.  6    is a flow chart  600  illustrating a method for substrate inspection, as discussed herein. The method, for example, inspects a first surface (e.g., the proximal surface) and a second surface (e.g., the distal surface) or near the second surface, e.g., using an inspection system, such as inspection system  200  shown in  FIGS.  2  and  4    or inspection system  500  shown in  FIG.  5   . 
     At block  602 , light is generated at a first wavelength directed at a substrate at a first angle, e.g., by a means for generating light at a first wavelength illustrated by the first light source  210  shown in  FIG.  2    or the first light source  510  shown in  FIG.  5   . 
     At block  604 , light is generated at a second wavelength directed at the substrate at a second angle that is an oblique angle with respect to a first surface of the substrate, e.g., by a means for generating light at a second wavelength illustrated by the second light source  220  shown in  FIG.  2    or the second light source  520  shown in  FIG.  5   . 
     At block  606 , the second angle is adjusted to focus the light at the second wavelength at a specified location, e.g., by a means for adjusting the second angle illustrated by the adjustment system  250  shown in  FIG.  2   . 
     At block  608 , backscattered light is collected from the substrate in response to the light at the first wavelength and the second wavelength, e.g., by a means for collecting backscattered light illustrated by the detection optics  230  shown in  FIG.  2    or the detection optics  530  shown in  FIG.  5   . 
     At block  610 , a first image representative of the first surface of the substrate is generated based on the backscattered light in a first spectrum, e.g., by a means for generating a first image illustrated by the first detector  216  and computing system  260  shown in  FIG.  2    and illustrated by image (defect map)  300  shown in  FIG.  3 A . 
     At block  612 , a second image representative of a second surface or near the second surface of the substrate is generated based on the backscattered light in a second spectrum, e.g., by a means for generating a second image illustrated by the second detector  226  and computing system  260  shown in  FIG.  2    and illustrated by image (defect map)  310  shown in  FIG.  3 B . 
     Based at least on the first image, defects on the first surface of the substrate may be detected, e.g., by the computing system  260 . Further, based at least on the second image, defects on the second surface (or near the second surface) may be detected, e.g., by the computing system  260 . In some implementations, based at least on the second image, defects in the bulk material between the first surface and the second surface (or near the second surface) may be detected, e.g., by the computing system  260 . In some implementations, defects in the second surface (or near the second surface), and/or defects in the bulk material between the first surface and the second surface (or near the second surface) may be detected using the second image and the first image. The defects detected on the first surface and the second surface (or near the second surface), and in some implementations, the bulk material between the first surface and the second surface, may be reported with respect to the substrate, e.g., stored in memory associated with the inspection location of the substrate or an indication of defects provided. 
     In some implementations, the substrate may be opaque to the light at the first wavelength and at least partially transparent to the light at the second wavelength, e.g., as discussed with reference to light  212  and  222  in  FIG.  2   . 
     In some implementations, the backscattered light from the substrate may be collected by receiving the backscattered light from the first surface and the backscattered light from the second surface or near the second surface with an elliptical mirror, e.g., as illustrated by the elliptical mirror  232  shown in  FIG.  2    or the elliptical mirror  532  shown in  FIG.  5   . Additionally, the backscattered light collected by the elliptical mirror may be directed towards at least one detector with at least one turning mirror comprising an aperture through which the light at the first wavelength is directed at the substrate at the first angle, e.g., as illustrated by the turning mirror  234  and aperture  235  shown in  FIG.  2    or the turning mirror  534  and aperture  535  shown in  FIG.  5   . In some implementations, the elliptical mirror may include a first aperture through which the light at the second wavelength is directed at the substrate at the second angle, and a second aperture through which light at the second wavelength that is reflected by the substrate exits the elliptical mirror, e.g., as illustrated by the elliptical mirror  532  and apertures  532   a  and  532   b  shown in  FIG.  5   . In some implementations, the light at the second wavelength may be directed at the substrate at the second angle between the elliptical mirror and the first surface of the substrate, e.g., as illustrated by light  222  and elliptical mirror  232  shown in  FIG.  2   . 
     In some implementations, the first angle is at or near a normal angle with respect to the first surface, as illustrated in  FIG.  2   . In some implementations, the first wavelength is a visible wavelength and the second wavelength is an infrared wavelength, and the backscattered light in the second spectrum is produced by the light at the second wavelength directed at the substrate at the second angle, as discussed in reference to  FIG.  2   . 
     In some implementations, the light at the first wavelength is focused at a first location at or near the first surface of the substrate and the second angle is adjusted by adjusting the second angle to focus the light at the second wavelength at the specified location at or near the second surface of the substrate that is on a z axis of the substrate shared with the first location, e.g., as illustrated by the adjustment system  250  shown in  FIG.  2    and discussed in  FIG.  4   . 
     In some implementations, a third image representative of defects on or near the second surface of the substrate corrected for residual signals of defects on the first surface may be generated based on a comparison of the first image and the second image, e.g., as discussed in reference to  FIG.  2    and shown by image (defect map)  320  shown in  FIG.  3 C . 
     This summary is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation. The detailed description is included to provide further information about the present patent application. 
     Each of the non-limiting aspects described herein or in one or more of the attached Appendices can stand on its own, or can be combined in various permutations or combinations with one or more of the other aspects or other subject matter described in this document. 
     The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific implementations that can be practiced. These implementations are also referred to generally as “examples.” Such examples can include elements in addition to those shown or described. However, examples are contemplated in which only those elements shown or described are provided. Moreover, examples are also contemplated using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein. 
     In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In this document, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following aspects, the terms “including” and “comprising” are open-ended, that is, a system, device, article, composition, formulation, or process that includes elements in addition to those listed after such a term in an aspect are still deemed to fall within the scope of that claim. Moreover, in the following aspects, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. 
     Method examples described herein can be machine or computer-implemented at least in part. Some examples can include a computer-readable medium or machine-readable medium encoded with instructions operable to configure an electronic device to perform methods as described in the above examples. An implementation of such methods can include code, such as microcode, assembly language code, a higher-level language code, or the like. Such code can include computer readable instructions for performing various methods. The code may form portions of computer program products. Further, in an example, the code can be tangibly stored on one or more volatile, non-transitory, or non-volatile tangible computer-readable media, such as during execution or at other times. Examples of these tangible computer-readable media can include, but are not limited to, hard disks, removable magnetic disks, removable optical disks (e.g., compact disks and digital video disks), magnetic cassettes, memory cards or sticks, random access memories (RAMs), read only memories (ROMs), and the like. 
     The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other implementations can be used, such as by one of ordinary skill in the art upon reviewing the above description. Also, in the above Description, various features may be grouped together to streamline the disclosure. The inventive subject matter may lie in less than all features of a particular disclosed implementation. Thus, the following aspects are hereby incorporated into the Description as examples or implementations, with each aspect standing on its own as a separate implementation, and it is contemplated that such implementations can be combined with each other in various combinations or permutations. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.