Abstract:
An apparatus for detecting top scattered light from a substrate. A source directs a light onto a position on the substrate. The light thereby reflects off in a specular beam, scatters off the top surface, and scatters off a bottom surface of the substrate. An objective receives the top and bottom scattered light. The objective has a first focal point focused on the position on the top surface of the substrate, and a second focal point focused on a pinhole field stop. The pinhole field stop passes the top scattered light that is focused on the pinhole field stop, and blocks the bottom scattered light. A sensor receives and quantifies the top scattered light.

Description:
FIELD 
     This application claims all rights and priority on copending U.S. provisional patent application Ser. No. 61/228,030 filed Jul. 23, 2009. This invention relates to the field of optical inspection technology. More particularly, this invention relates to the optical inspection of transparent substrates and epitaxial layers deposited upon transparent substrates. 
    
    
     INTRODUCTION 
     Transparent substrates, such as silicon carbide and sapphire, are frequently used in the fabrication of light emitting diodes. Such transparent substrates are often polished on only a single side of the substrate. In such substrates, the upper active surface is polished and the lower inactive surface remains unpolished. If a conventional scatterometer is used to inspect the substrate, then it is difficult to inspect the polished upper surface. The laser beam from the scatterometer penetrates the transparent substrate and strikes the bottom unpolished surface. The scatter signal from the unpolished bottom surface typically overwhelms the signal from the defects on the top surface. As a result, it is difficult to detect any defects that might be present on the top surface of the substrate. 
     What is needed, therefore, is a system that overcomes problems such as those described above, at least in part. 
     SUMMARY OF THE CLAIMS 
     The above and other needs are met by an apparatus for detecting top scattered light from a top surface of a substrate. A light source directs a beam of light onto a focal position on the top surface of the substrate, where the substrate is at least partially transmissive to the beam of light. The beam of light thereby, (a) specularly reflects off of the top surface of the substrate to produce a specular beam, (b) scatters up off of the focal position on the top surface of the substrate to produce the top scattered light, and (c) refracts into the substrate and scatters up off of a bottom surface of the substrate to produce bottom scattered light. An objective receives the top scattered light and an unblocked portion of the bottom scattered light. The objective has a first focal point focused on the focal position on the top surface of the substrate, and a second focal point focused on a pinhole field stop. The pinhole field stop receives the top scattered light and the unblocked portion of the bottom scattered light, and passes the top scattered light that is focused on the pinhole field stop, and blocks the unblocked portion of the bottom scattered light. A sensor disposed opposite the objective across the pinhole field stop receives and quantifies the top scattered light. 
     In this manner, the apparatus according to the various embodiments of the present invention separates the scattered light from the bottom surface of the substrate from the scattered light from the top surface of the substrate before the scattered light from the bottom surface of the substrate can attain the sensor. Thus, the signal from the sensor can be analyzed for defects on the top surface of the substrate, without being confounded or overpowered by the scattered light from the bottom surface of the substrate. 
     In various embodiments according to this aspect of the invention, the beam of light is a laser beam. In some embodiments the beam of light is directed obliquely onto the focal position on the top surface of the substrate. In other embodiments the beam of light is directed onto the focal position normal to the top surface of the substrate. In some embodiments the substrate is formed of at least one of silicon carbide and sapphire. In some embodiments the top surface of the substrate is relatively polished compared to the bottom surface of the substrate, and the bottom surface of the substrate is relatively rough compared to the top surface of the substrate. In some embodiments the objective is an ellipsoid of revolution having a mirror-polished interior surface. In other embodiments the objective is an aberration-corrected microscope objective. In still other embodiments the objective is a reflective microscope objective having a primary spherical mirror and secondary spherical mirror. In some embodiments a beam block disposed at least one of adjacent or interior to the objective for blocking at least a portion of the bottom scattered light. In some embodiments the objective has a field of view of about five hundred microns and a numerical aperture of about 0.52. 
     According to another aspect of the invention there is described a method for detecting top scattered light from a top surface of a substrate. A beam of light is directed onto a focal position on the top surface of the substrate, where the substrate is at least partially transmissive to the beam of light. The beam of light thereby (a) specularly reflects off of the top surface of the substrate to produce a specular beam, (b) scatters up off of the focal position on the top surface of the substrate to produce the top scattered light, and (c) refracts into the substrate and scatters up off of a bottom surface of the substrate to produce bottom scattered light. The top scattered light and an unblocked portion of the bottom scattered light are received with an objective. The objective has a first focal point focused on the focal position on the top surface of the substrate, and a second focal point focused on a pinhole field stop. The top scattered light and the unblocked portion of the bottom scattered light are received with the pinhole field stop, which passes the top scattered light that is focused on the pinhole field stop and blocks the unblocked portion of the bottom scattered light. A sensor disposed opposite the objective across the pinhole field stop receives and quantifies the top scattered light. 
     In various embodiments according to this aspect of the invention, the beam of light is a laser beam. In some embodiments the beam of light is directed obliquely onto the focal position on the top surface of the substrate. In other embodiments the beam of light is directed onto the focal position normal to the top surface of the substrate. In some embodiments the objective is an ellipsoid of revolution having a mirror-polished interior surface. In other embodiments the objective is an aberration-corrected microscope objective. In still other embodiments the objective is a reflective microscope objective having a primary spherical mirror and secondary spherical mirror. In some embodiments at least a portion of the bottom scattered light is blocked with a beam block disposed at least one of adjacent or interior the objective. In some embodiments an output of the sensor is analyzed to detect defects on the top surface of the substrate. 
     According to yet another embodiment of the present invention there is described an apparatus for detecting top scattered light from a top surface of a transparent substrate, where the top surface of the substrate is relatively polished compared to the bottom surface of the substrate, and the bottom surface of the substrate is relatively rough compared to the top surface of the substrate. A light source obliquely directs a laser beam onto a focal position on the top surface of the substrate. The laser beam thereby (a) specularly reflects off of the top surface of the substrate to produce a specular beam, (b) scatters up off of the focal position on the top surface of the substrate to produce the top scattered light, and (c) refracts into the substrate and scatters up off of a bottom surface of the substrate to produce bottom scattered light. 
     An objective receives the top scattered light and an unblocked portion of the bottom scattered light. The objective has a first focal point focused on the focal position on the top surface of the substrate, and a second focal point focused on a pinhole field stop. The objective is a reflective microscope objective having a primary spherical mirror and secondary spherical mirror, with a field of view of about five hundred microns and a numerical aperture of about 0.52. A beam block is disposed at least one of adjacent or interior to the objective, and blocks at least a portion of the bottom scattered light. The pinhole field stop receives the top scattered light and the unblocked portion of the bottom scattered light, and passes the top scattered light that is focused on the pinhole field stop and blocks the unblocked portion of the bottom scattered light. A sensor is disposed opposite the objective across the pinhole field stop, and receives and quantifies the top scattered light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
         FIG. 1  depicts a collector objective for a scatterometer according to a first embodiment of the present invention. 
         FIG. 2  depicts a collector objective for a scatterometer according to a second embodiment of the present invention. 
         FIG. 3  depicts a collector objective for a scatterometer according to a third embodiment of the present invention. 
         FIG. 4  depicts a light pattern from a collector objective for a scatterometer according to the third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference now to  FIG. 1 , there is depicted an embodiment  100  of the present invention, having an ellipsoidal collector  132  with an internal beam block  130  and field stop (pinhole)  126 , used as a scattered light collector. The ellipsoidal collector  132 , internal beam block  130 , and field stop  126  provide a degree of isolation between the light  122  that is scattered off of the top surface  116  of the substrate  118 , and the light  124  that is scattered off of the bottom surface  120  of the substrate  118 . 
     An input laser beam  112  is directed beneath an internally-reflective ellipsoid of revolution  132 . A portion of the input laser beam  112  specularly reflects off of the top surface  116  of the substrate  118  as a reflected beam  114 , and a portion  122  of the input laser beam  112  is scattered from the top surface  116  into the ellipsoid  132 . A portion of the input laser beam  112  refracts into the transparent substrate  118  and is scattered off of the unpolished bottom surface  120  of the substrate  118  as scattered light  124 . The substrate  118  is formed of a material such as silicon carbide or sapphire. 
     Ellipsoid  132  is located with its first or lower foci at the top surface  116  of the transparent substrate  118 , at the point where the input laser beam  112  reflects from the upper surface  116  of the substrate  118 . The pinhole or field stop  126  is located at the second or upper foci of the ellipsoid  132 . The portion of the scattered light  124  from the back surface  120  that does not reflect from the mirrored internal surfaces of the ellipsoid of revolution  132  is blocked by an absorbing beam block  130 , placed internally to the ellipsoid  132 . The scattered light  122  that originates from the lower foci is thus directed to the second foci and passes through the pinhole  126 , and is received by a photomultiplier tube  128 , or some other light sensor. 
     With reference to  FIG. 2 , there is depicted a second embodiment  200  of an aberration-corrected microscope objective  232  that is used to collect the scattered light  222  from the top surface  216 . Laser light  212  is brought in beneath the objective  232 . A portion  214  specularly reflects from the top surface  216 , and a portion  222  scatters from the top surface  216 . Another portion of the beam  212  refracts into the transparent substrate  218  and scatters from the bottom unpolished surface  220  as scattered light  224 . The scattered light  224  from the bottom surface  220  is partially blocked by an absorbing beam block  230  placed beneath the objective  232 . The objective  232  is focused at the top surface  216  at the point where the laser beam  212  reflects off the surface  216  of the substrate  218 . 
     When the scattered light  224  from the bottom surface  220  appears at the upper conjugate of the objective  232 , it is shifted to the left (in the embodiment as depicted). A pinhole  226  placed at the upper conjugate separates the top scattered light  222  and the bottom scattered light  224 . The scattered light  222  from the top surface  216  passes through the pinhole  226  and is detected by the sensor  228 . 
     Because the microscope objective  232  is an imaging device with a relatively wider field of view than the ellipsoid  132 , the scattered light  224  from the bottom surface  220  simply appears as an out of focus spot with a central hole (due to the presence of beam absorber  230 ). In this manner, the microscope objective  232  is quite successful in separating the light  222  scattered from the top surface  216  and the light  224  scattered from the bottom surface  220 . 
     The operation of the beam block  230  is further clarified in an embodiment where a normally incident beam (not depicted) is directed onto and scatters from the top surface  216  and the bottom surface  220 . The scattered light  222  from the top surface  216  follows the path through the pinhole  226 . The scattered light  224  from the bottom surface  220  follows the same path, except that it appears as an out of focus spot with a central hole surrounding the focused beam  222  at the location of the pinhole  226 . The central hole is created by the absorbing beam block  230 . The pinhole  226  only passes the scattered light  222  from the top surface  216 . It is the presence of the beam block  230 , combined with the imaging nature of the microscope objective  232 , that allows this separation of the light  224  scattered from the bottom surface  220  and the light  222  scattered from the top surface  216 . 
     With reference now to  FIG. 3 , there is depicted an embodiment  300  of the present invention. A laser beam  312  is directed onto the top surface  316  of the substrate  318 , and specularly reflects off of the top surface  316  of the substrate  318  as reflected beam  314 . Some of the laser beam  312  scatters from the polished top surface  316  of the substrate  318  as scattered light  322 , and some of the laser beam  312  refracts into the substrate  318  and scatters from the unpolished bottom surface  320  as scattered light  324 . 
     The scattered lights  322  and  324  are received by a reflective microscope objective composed of a primary spherical mirror  332 , and are reflected onto a secondary spherical mirror  330 , before passing through the port  334  in the primary mirror  332 . Secondary spherical mirror  330  is designed to obstruct less than about twenty percent of the total light scattered from the surface so as to optimize the sensitivity of the system  300 . In one embodiment, the objective  332  has a five hundred micron field of view with a 0.52 numerical aperture and a long working distance. The focal point of the objective  332  is located at the upper surface  316  at a point where the laser beam  312  reflects off of the upper surface  316 . 
     Once the light  322  and the light  324  are outside of the port  334 , the scattered light  322  from the top surface  316  passes through the pinhole field stop  326  to attain the light sensor  328 , while the scattered light  324  from the unpolished backside  320  of the substrate  318  comes at a different angle and is blocked by the pinhole field stop  326  from attaining the light sensor  328 . 
     It is sometimes desirable to bring in laser light at different angles of incidence to the substrate  318 . System  300  also depicts a normally-incident beam  313  that reflects off of a forty-five degree turning mirror  305  before striking the surface  316 . In this embodiment, the scattered light  324  from the bottom surface  320  is not directed to the left or right of the pinhole  326 , but appears as a bright spot with a hole in its center caused by the secondary mirror  330 . The hole is centered on the pinhole  326 , which separates the scattered light  322  from the top surface  316  from the scattered light  324  from the bottom surface  320  of the transparent substrate  318  in that embodiment. 
     It is appreciated that embodiment  300  is optimized for separating the scattered light  322  from the top surface  316  from the scattered light  324  from the bottom surface  320  of the transparent substrate  318 . This embodiment  300  uses an all-reflective optical design, where no bulk scatter is generated and only two surfaces (the primary mirror  332  and the secondary mirror  330 ) contribute to the residual scatter due to surface roughness. 
     By way of further explanation,  FIG. 4  depicts the scattered light patterns that appear on the plane of the pinhole  326 . The focused scattered light  322  from the top surface  316  of the substrate  318  appears at the center of the pinhole  326 , surrounded by the defocused scattered light  324   a  from the bottom surface  320  of the substrate  318  from the normally incident beam  313 . 
     The hole  402  in the scattered light  324   a  is created by the secondary mirror  330 . The cross members  404  in the scattered light  324   a  (depicted by way of example) are caused by the support structures (not depicted in  FIG. 3 ) that hold the secondary mirror  330  in place. The scattered light pattern  324   b  to the left of the pinhole  326  is created by the scattered light  324  from the bottom surface  320  that originates from the oblique beam  312 . In this embodiment, the pattern  342   b  is a defocused spot with a hole  402  surrounded by cross members  404 . 
     In some instances it is desired to use different wavelengths for the normal beam  313  and the oblique beam  312 . In such an embodiment, the reflective objective  332  is particularly advantageous because it exhibits no chromatic aberration. 
     The foregoing description of embodiments for this invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.