PRIORITY 
       [0001]    The present application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 61/982,754, filed Apr. 22, 2014, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present disclosure is directed generally toward wafer inspection systems, and more particularly toward line scan inspection devices with confocal scanning elements. 
       BACKGROUND 
       [0003]    In confocal microscopy, a pinhole filter in the confocal plane of the lens eliminates unfocussed light. However, because confocal pinhole filters clip illumination lines projected to the sensor; any shift of that line relative to the pinhole filter will result in a change of the image intensity. 
         [0004]    While improving resolution, confocal pinhole filters also increase tool sensitivity to focus and boresight variation due to thermal drifts, vibrations and mechanical repeatability of components. Increased sensitivity affects stability of the tool and eliminating all of the increased sensitivity factors is impractical due to tool complexity. 
         [0005]    Consequently, it would be advantageous if an apparatus existed that is suitable for wafer inspection with confocal slit aperture filters and increase stability. 
       SUMMARY 
       [0006]    Accordingly, the present invention is directed to a novel apparatus for wafer inspection with confocal slit aperture filters and increase stability. 
         [0007]    In one embodiment, a line scan wafer inspection system includes a detector associated with each slit aperture filter in the confocal plane to monitor and correct line positions relative to the slit aperture filter to keep image intensity variations within tolerable limits. Each detector measures a line position and then uses the line position signal to adjust optical components in the collection path in a feedback loop. 
         [0008]    In one embodiment, the feedback loop is employed in a runtime calibration process. In another embodiment, the feedback loop is employed as a real-time compensation mechanism during inspection to enhance stability. 
         [0009]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The numerous advantages of the present disclosure may be better understood by those skilled in the art by reference to the accompanying figures in which: 
           [0011]      FIG. 1A  shows a front view of a wafer inspection system according to one embodiment of the present disclosure; 
           [0012]      FIG. 1B  shows a detailed top view of one image sensor and position detector pair; 
           [0013]      FIG. 2  shows a close-up representation of wafer light scattered in a moving wafer inspection system; 
           [0014]      FIG. 3  shows a close-up of a portion of the wafer inspection system of  FIG. 1 ; 
           [0015]      FIG. 4  shows a close-up of a portion of the wafer inspection system of  FIG. 1 ; 
           [0016]      FIG. 5  shows a block diagram of a computer system for implementing embodiments of the present disclosure; and 
           [0017]      FIG. 6  shows a flowchart for a method of adjusting inspection optics in a line illumination wafer inspection system. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. The scope of the invention is limited only by the claims; numerous alternatives, modifications and equivalents are encompassed. For the purpose of clarity, technical material that is known in the technical fields related to the embodiments has not been described in detail to avoid unnecessarily obscuring the description. 
         [0019]    An understanding of one or more embodiments of the present disclosure may be further illuminated by U.S. Pat. No. 7,525,649 which is hereby incorporated by reference. 
         [0020]    Referring to  FIG. 1A , a front view of a wafer inspection system according to one embodiment of the present disclosure is shown. In some embodiments, a wafer inspection system, such as a line scan optical inspection system  100  includes three collection channels  102 ,  104 ,  106  to collect scattered light from defects in a wafer  100  and image the light onto corresponding array line sensors  114 ,  116 ,  118 . A position detector  115 ,  117 ,  119  is associated with each array line sensor  114 ,  116 ,  118 . In some embodiments, illumination optics form a thin line  120  at a scanning location on the wafer  101 . Scattered light from the illuminated scanning location on the wafer  101  is gathered and focused by the three collection channels  102 ,  104 ,  106  onto each array line sensor  114 ,  116 ,  118 . In one exemplary embodiment, the thin line  120  may have, but is not required to have, a width less than 1 micrometer on the wafer. Scattered light from wafer  101  features and defects are imaged onto the array line sensors  114 ,  116 ,  118  in each collection channel  102 ,  104 ,  106 . The array line sensors  114 ,  116 ,  118  may include any imaging technology known in the art. In some embodiments, array line sensors  114 ,  116 ,  118  include charge coupled devices (CCD) or time-delayed integration (TDI) devices. 
         [0021]    In some embodiments, each array line sensor  114 ,  116 ,  118  is associated with a slit aperture filter  108 ,  110 ,  112 . In at least one embodiment, where the line scan optical inspection system  100  includes a line illumination device, the array line sensors  114 ,  116 ,  118  may be configured for a large field of view in the x-direction (perpendicular to the illuminated line  120 ) to collect all scattered light. The position detectors  115 ,  117 ,  119  are positioned just outside of the corresponding array line sensor  114 ,  116 ,  118  in y-direction (along the long axis of the illuminated line  120 ). These position detectors  115 ,  117 ,  119  tracks light scattered from wafer  101  and compare the position of the scattered light against a calibrated position corresponding to the center of a corresponding slit aperture filter  108 ,  110 ,  112 . An error signal generated by the position detectors  115 ,  117 ,  119  is used to move an optical element in the corresponding collection channel  102 ,  104 ,  106  to ensure scattered light from wafer  110  is centered on the slit aperture filter  108 ,  110 ,  112 . A person skilled in the art may appreciate that while the position detectors  115 ,  117 ,  119  in  FIG. 1  are shown offset the array line sensors  114 ,  116 ,  118  in a direction perpendicular to the illumination line  120 , such illustration is merely a function of the limitations of a two-dimensional medium. 
         [0022]    In some embodiments, slit aperture filters  108 ,  110 ,  112  substantially abut the corresponding array line sensors  114 ,  116 ,  118 . In other embodiments, where the slit aperture filters  108 ,  110 ,  112  are separated from the corresponding array line sensors  114 ,  116 ,  118  by some distance, a cylinder lens may re-focus light to the slit aperture filters  108 ,  110 ,  112 . 
         [0023]    For line scan inspection tools, the resolution in the wafer  101  scanning direction may be determined by an illumination line profile. Resolution in the x-direction may be determined by the illumination line width. The line width may be limited by the numerical aperture (NA) of the line formation cylinder (LFC) which has a theoretical limit of  1 , and by the Gaussian beam at the entrance pupil. It is noted that a smaller line width may be achieved with a more aggressive Gaussian fill factor, but sidelobes from diffraction ringing create a performance limitation. A confocal slit aperture filter at the detector enhances the resolution in the scanning direction beyond this limitation. In a line scan optical inspection system  100 , a confocal slit aperture filter  108 ,  110 ,  112  at each array line sensor  114 ,  116 ,  118  enhances the resolution in the scanning direction and suppresses sensitivity to illumination line sidelobes. 
         [0024]    When a point scans across the wafer  101  in the direction perpendicular to the illumination line (x-direction), the image produced on an array line sensor  114 ,  116 ,  118  can be described using a point spread function defined for each collection channel  102 ,  104 ,  106 : 
         [0000]      F PS      —     Channel (X, Y, Z) 
         [0000]    where X, Y are local coordinates for each array line sensor  114 ,  116 ,  118  and Z is a defocus value when the illuminated point is not at the array line sensor  114 ,  116 ,  118  conjugate. The illumination line spread function is described by: 
         [0000]      F PS     —     Illumination (xw) 
         [0000]    at the wafer coordinate xw. 
         [0025]    Given a magnification M for a collection channel  102 ,  104 ,  106 , the overall point spread function, including the illumination profile is: 
         [0000]        F   PS ( xw, yw, X )= F   PS     —     Illumination ( xw )* F   PS     —     Channel ( X−M*xw, M*yw, Z ) 
         [0000]    Integrated over the length of the array line sensor  114 ,  116 ,  118  over X. 
         [0026]    For a center collection channel  104 , Z is constant, with a narrow slit aperture filter  110 , centered at X=0: 
         [0000]        F   PS ( xw, yw )= F   PS     —     Illumination ( xw )* F   PS     —     Center (− M*xw, M*yw, Z )
 
         [0000]    In one embodiment, if both point spread functions have a Gaussian shape, exp(−X 2 /W 2 ), the system point spread function is also Gaussian, with a width W (W IL  of the illumination point spread function and W C  channel point spread function), calculated as: 
         [0000]      1/ W   2 =1/ W   IL     2   +1/ W   C     2      
         [0000]    Such a point spread function may have a smaller line width and higher resolution. With large slit aperture filter  108 ,  110 ,  112  width, after integration of the channel point spread function, the line width in the x-direction is determined by the illumination line width only. It is noted that at least in some instances a narrow slit aperture filter  108 ,  110 ,  112  will not collect sidelobe energy, but a wide slit aperture filter  108 ,  110 ,  112  will collect sidelobe energy. 
         [0027]    It is further noted that a similar effect is observed in the case of a side collection channel  114 ,  118 . In this case, for large slit aperture filter  108 ,  110 ,  112  and detector  114 ,  116 ,  118  width, the overall line width is determined by the illumination line width. Further, for a slit aperture filter  108 ,  110 ,  112 , line width is reduced because of a multiplication factor. 
         [0028]    When a narrow slit aperture filter  108 ,  110 ,  112  is implemented, either mechanically or electronically at the sensor, sidelobes can be suppressed significantly. Suppressing sidelobes allows for higher resolution with smaller line width. Properly suppressing sidelobes requires the focused light to be correctly centered on the slit in the slit aperture filter  108 ,  110 ,  112 ; therefore position detectors  115 ,  117 ,  119  that allow the line illumination system  100  to alter the position of optical elements in one or more of the collection channels  102 ,  104 ,  106  is desirable. However, inspection sensitivity may suffer because a slit aperture filter  108 ,  110 ,  112  will necessarily cause a degree of light loss. It is, therefore, important that the slit aperture filter  108 ,  110 ,  112  be replaceable when more light is needed. A system including mechanical slit aperture filters  108 ,  110 ,  112  may include a mechanism for swapping such slit aperture filters. Alternatively, a system including electronic slit aperture filters  108 ,  110 ,  112  may be configured such that the electronic slit aperture filters may be adjusted with different slit aperture widths for different applications. 
         [0029]    Referring to  FIG. 1B , a detailed top view of one image sensor and position detector pair is shown. In one exemplary embodiment, the first collection channel of  FIG. 1A  has an associated image sensor  116  and position detector  117 . An illumination line image  122  from the first collection channel illuminates both the image sensor  116  and position detector  117 . The portion of the illumination line image  122  that illuminates the image sensor would be filtered by a corresponding slit aperture filter  108  while the portion illuminating the position detector would not. The position detector  117  produces a signal corresponding to the position of the illumination line image  122  in the scanning direction (x-direction). In some embodiments, similar image sensor  116 /position detector  117  pairs are used for each collection channel. 
         [0030]    Referring to  FIG. 2 , a close-up representation of wafer light scattered in a moving wafer inspection system is shown. In a wafer inspection process, a wafer is illuminated by a line illumination source having an illumination profile  206  as the wafer moves in a direction of travel  200 . Primary light  210  is scattered by the wafer to be received and focused by one or more collection channels. Furthermore, secondary light  212  may be scattered by undesirable structures  202 . 
         [0031]    Referring to  FIG. 3 , a close-up of a portion of the wafer inspection system of  FIG. 1  is shown. In at least one embodiment of the present disclosure, a first slit aperture filter  108  is associated with a first collection channel. The first slit aperture filter  108  is positioned and oriented in the confocal plane as defined by the first collection channel optics. The first slit aperture filter  108  transmits a primary beam  300  associated with the primary light scattered by the wafer by the line illumination source, and filters out incidental beams  302  associated with the secondary light or sidelobes of the line illumination beam scattered by the wafer. 
         [0032]    In at least one embodiment, the first slit aperture filter  108  comprises a mechanical filter. The mechanical filter may be replaceable with mechanical filters having larger slit apertures to allow more light where increased sensitivity is required. Alternatively, in at least one embodiment, the first slit aperture filter  108  comprises an electronic filter. The electronic filter may be adjustable to produce a larger or smaller aperture as desired for system sensitivity for particular applications. 
         [0033]    Referring to  FIG. 4 , a close-up of a portion of the wafer inspection system of  FIG. 1  is shown. In at least one embodiment of the present disclosure, a second slit aperture filter  110  is associated with a second collection channel. The second slit aperture filter  110  is positioned and oriented in the confocal plane as defined by the second collection channel optics. The second slit aperture filter  110  transmits a primary beam  400  associated with the primary light scattered by the wafer by the line illumination source, and filters out incidental beams  402  associated with the secondary light or sidelobes of the line illumination beam scattered by the wafer. 
         [0034]    In at least one embodiment, the second slit aperture filter  108  comprises a mechanical filter. The mechanical filter may be replaceable with mechanical filters having larger slit apertures to allow more light where increased sensitivity is required. Alternatively, in at least one embodiment, the second slit aperture filter  108  comprises an electronic filter. The electronic filter may be adjustable to produce a larger or smaller aperture as desired for system sensitivity. 
         [0035]    Referring to  FIG. 5 , a block diagram of a computer system  500  for implementing embodiments of the present disclosure is shown. The computer system  500  includes a processor  502 , memory  504  connected to the processor  502  for embodying processor executable program code, and one or more detectors  508 ,  510 ,  512  connected to the processor  502 . In some embodiment, each of the one or more detectors  508 ,  510 ,  512  are associated with a collection channel. In some embodiments, the computer system  500  includes a data storage element  506  connected to the processor  502 . In some embodiments, the data storage element  506  is configured to store one or more illumination profiles and one or more line images received from the one or more detectors  508 ,  510 ,  512 . 
         [0036]    In some embodiments, the computer system  500  may further include an electronic aperture  514 ,  516 ,  518  associated with each detector  508 ,  510 ,  512  and connected to the processor  502 . In some embodiments, the electronic apertures  514 ,  516 ,  518  may be adjustable to produce larger or smaller slit apertures as necessary. 
         [0037]    Referring to  FIG. 6 , a flowchart depicting a method of adjusting inspection optics in a line illumination wafer inspection system is shown, in accordance with an embodiment of the present disclosure. In a first step  600 , in an inspection system, a wafer illumination line image from one or more collection channels is filtered with a corresponding slit aperture. In a second step  602 , one or more detectors receive the illumination line image. 
         [0038]    In a third step  604 , a processor connected to the one or more detectors analyzes the illumination line image to determine if the collection channel is configured to focus the illumination line image at a predetermined confocal plane position. In a fourth step  606 , the processor adjusts the confocal plane of the collection channel. The confocal plane may be adjusted by altering the relative or absolute positions of one or more optical elements in the collection channel, altering a mechanical element in the line illumination wafer inspection system performing the wafer inspection processes, altering an electronic or optical element in the illumination device creating the thin line illumination, or altering an electrical component in the line illumination wafer inspection system performing the wafer inspection process. 
         [0039]    Alternatively, a processor connected to the one or more detectors will analyze  608  the illumination line image to determine if the illumination line image at the detector is at a desired position. The processor may then adjust  610  the position of the slit aperture, the position of one or more optical elements in a corresponding collection channel, a mechanical element in the line illumination wafer inspection system performing the wafer inspection processes, an electronic or optical element in the illumination device creating the thin line illumination, or an electrical component in the line illumination wafer inspection system performing the wafer inspection process to move the illumination line image to the desired position. 
         [0040]    In any embodiment, the inspection system may filter  600  and receive  602  subsequent illumination line images in a feedback loop adjustment process. 
         [0041]    It is believed that the present invention and many of its attendant advantages will be understood by the foregoing description of embodiments of the present invention, and it will be apparent that various changes may be made in the form, construction, and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.
A line scan wafer inspection system includes a confocal slit aperture filter to remove sidelobes and enhance resolution in the scanning direction. A position detector associated with the slit aperture filter monitors and corrects illumination line image positions relative to the slit aperture to keep image position variations within tolerable limits. Each detector measures a line position and then uses the line position signal to adjust optical, mechanical, and electronic components in the collection path in a feedback loop. The feedback loop may be employed in a runtime calibration process or during inspection to enhance stability.