Abstract:
A problem in the inspection of transparent wafers and disks is the detection of top surface particles. More precisely, it is being able to assign a scattering site as being due to a particle at the top or bottom surface of a transparent wafer. A method of the present invention is to use an elliptical mirror, with a pinhole at its top focus, together with a focused beam. The focused beam will diverge as it passes through the transparent wafer and as a result any particle on the bottom surface will see a lower optical intensity and will appear weaker than a top surface particle. The suppression of scattered light from the bottom surface occurs because the source of the scattered light (the bottom surface) is far from the bottom foci of the elliptical mirror. This means that the light from the bottom surface, which arrives inside the ellipsoid, will be out of focus at the top foci of the ellipsoid and as a result very little light from the bottom surface will pass through the pinhole at the top foci of the elliptical mirror. This reduction of light from the bottom surface can be further improved by making the pinhole diameter to be substantially less than the thickness of the transparent wafer.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to the field of surface analyzers and more particularly to transparent wafer or disk inspection. 
   2. Description of Background Art 
   A problem in the inspection of transparent wafers is the detection of top surface defects without detecting defects from the bottom side of the transparent wafers. Conventional methods for detecting particles (defects) within or upon the top surface of a transparent wafer rely upon illuminating a sample with a beam of light that has been channeled through a modest numerical aperture lens. In these systems, the incident light expands once it leaves the focal point and the intensity of the light decreases as the beam passes through the transparent wafer. The incoming light produces a significant scattering signal when interacting with surface particles and near surface defects on the transparent wafer, while the scattering signal from sub-surface and back-surface defects is substantially reduced by the divergence of the incoming beam (reduction in beam intensity). These types of methods are limited to investigation of relatively thick wafers (on the order of 2.0 mm), and the divergence of the beam does not eliminate the signal from the back surface it only reduces it. When analyzing thin transparent wafers with a modest numerical aperture system, the beam which passes through the wafer has sufficient intensity that it becomes difficult to distinguish whether a defect is on the top or bottom of a transparent wafer. Other conventional defect detection systems provide a wavelength of light for illumination that is selected to match an area where the transparent wafer appears opaque, thus minimizing the influence of light scattering from the subsurface or back side of the transparent material. However, many defects are left undetected due to a lack of penetration depth from the incident light. Another problem occurs when transparent wafers have only their top surface polished. The beam which penetrates through the transparent wafer and strikes the bottom (unpolished) surface generates a large scattered light signal. This large scattered light signal from the bottom surface obscures the scattered light from defects on the top surface making top surface defect detection difficult or impossible. 
   What is needed is a system and method that (1) analyzes transparent wafers using a single sided system, (2) distinguishes the position of scattering sites, (3) measures the effects of scattering only from scattering sites located within an defined depth of the top surface of the wafer, and (4) permits modification of the depth of the scattering sites that are measured. 
   SUMMARY OF THE INVENTION 
   The present invention analyzes transparent wafers using a single sided system, distinguishes the position of scattering sites, measures the effects of scattering only from scattering sites located within a defined depth of the top surface of the wafer, and permits modification of the depth of the scattering sites that are measured. An input light beam is focused onto a surface of a transparent wafer. The light penetrates the surface to a scattering depth where the light scatters from a particular scattering site (particle or defect). The scattered light is then collected by an elliptical (or ellipsoidal) mirror and restricted by a pinhole before being channeled into a light sensor for further analysis. The scattering depth is adjusted by varying the diameter of the pinhole to match the depth of focus needed to resolve scattering sites substantially near the top surface of the transparent wafer. The bottom surface of a transparent wafer may also be inspected. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an illustration of a system for detecting particles on only the top surface of a transparent surface in accordance with an embodiment of the present invention. 
       FIG. 2  is an illustration of a system for detecting particles on only the bottom surface of a transparent surface in accordance with an embodiment of the present invention. 
       FIG. 3  is a flowchart illustrating a method for detecting particles on only one side of a transparent surface in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An embodiment of the present invention can be described as a confocal scatterometer. A traditional confocal microscope uses the specular return from the sample to form an image. In contrast an embodiment of the invention detects the scattered light return from the sample. The present confocal scatterometer allows for the imaging of defects or particles on top of a transparent surface without interference from the bottom surface. Alternatively, another embodiment may image bottom surface defects or particles without interference from the top surface. In another embodiment, it is possible to image defects within the body of the transparent material by the use of a sufficiently small diameter pinhole and by focusing the ellipsoid to a point within the transparent material. 
     FIG. 1 , in accordance with an embodiment of the present invention, illustrates a system  100  for detecting particles on one side of a transparent surface. In an embodiment of the present invention, the transparent surface is a transparent wafer  106  with a top surface  124 , a bottom surface  126 , and has a thickness “t”  130 . The transparent object  106  can be selected from a variety of materials including at least: a semiconducting wafer, a silicon carbide wafer, a sapphire wafer, a lithium niobate wafer, a glass substrate, a lithium tantalate wafer, a gallium nitride wafer, a gallium arsenide wafer or a silicon wafer. An elliptical or ellipsoidal mirror  110 , hereafter referred to as an elliptical mirror for convenience, is provided to collect the scattered light from the transparent wafer  106 . The ellipsoidal mirror  110  is a body of revolution of an ellipse. In one embodiment, the top and bottom of the ellipsoidal mirror  110  are removed (not shown) and the top focus  102  of the elliptical mirror  110  is positioned within a pinhole  108 , of diameter d, within an opaque surface  121  that passes perpendicular to the ellipsoid  110  and contains the top focus  102 . The bottom focus  128  of the elliptical mirror  110  is positioned on the top surface  124  of the transparent wafer  106 . The optical beam  104  (generated from a 10 microns to 193 nm laser or another optical source) comes to a focus at the bottom focus  128  of the ellipsoid  110  and strikes the top surface  124  of the transparent wafer  106 . 
   When the incoming beam  104  interacts with a scattering site (not shown) on the top surface  124  of the transparent wafer  106 , the first scattered beam  114  propagates through elliptical mirror  110 . A portion of the incoming beam penetrates the top surface  124  of transparent wafer  106  and a second scattered beam  116  propagates through the elliptical mirror  110  due to interaction with a second scattering site (not shown) at a particular scattering depth. In an embodiment of the present invention ( FIG. 1 ), the second scattering sight is located on the bottom surface  126  of the transparent wafer  106  and the detectable scattering depth is equal to the thickness  130  of the transparent wafer  106 . In another embodiment of the present invention (not shown), the second scattering site is located between the top surface  124  and bottom surface  126  of the transparent wafer  106  and the detectable scattering depth is less than the thickness  130  of the transparent wafer  106 . The thickness  130  of the transparent wafer  106 , according to an embodiment of the present invention, is approximately 250 micrometers. 
   The first scattered beam  114  is channeled to a focus at a position that is substantially at the top focal point  102  of elliptical mirror  110  in order to pass through the pinhole  108  of opaque surface  121  undisturbed. The first scattered beam  114  is directed onto scatter sensor  111  for further analysis. 
   Another portion of incoming beam  104  transverses through the top surface  124  and bottom surface  126  of transparent wafer  106  as transmitted beam  120 . Another portion of incoming beam  104  transverses through the top surface  124  of transparent wafer  106  and is reflected, as second scattering beam  116 , from the bottom surface  126  of the transparent wafer  106 . A first portion of second scattering beam  116  propagates through elliptical mirror  110  and is restricted from entering scatter sensor  111  by the diameter d of the pinhole  108  in opaque surface  121 . A portion  112  of the incident beam  104  reflects from the top surface of the transparent wafer  124  and exits through an opening in the side of elliptical mirror  110 . As shown in U.S. patent application Ser. No. 10/754,275, which is incorporated herein by reference in its entirety, a more detailed analysis of surface particles and defects is available by collecting and analyzing the reflected portion  112  of incident beam  104  with a specular sensor. 
   According to an embodiment of the present invention, the diameter d of the pinhole  108  in the opaque surface  121  is selected at a value substantially less than the thickness t  130  of the transparent wafer  106 . When the diameter d of the pinhole  108  (or two times the depth of focus) is substantially less than the thickness  130  of the transparent wafer  106  and the bottom focus  128  of the elliptical mirror  110  is on the top surface  124 , then defects on the top surface  124  are visible in the scattered light detected at scatter sensor  111 . Selecting a pinhole diameter up to 70% smaller than the thickness of the transparent wafer is sufficient to distinguish particles on the top and bottom surfaces. For the case that the thickness  130  of the transparent wafer  106  is 250 μm, a pinhole diameter of 150 μm is adequate for distinguishing between top and bottom surface particles. In another embodiment, the diameter d may be chosen to approximately equal the thickness t  130  of the transparent wafer. This embodiment allows the collection of scattered light from both the top  124  and bottom surfaces  126  of the transparent wafer. 
   The combination of a divergent incoming beam  104  (as it passes through the transparent wafer  106 ) with an elliptical mirror  110 , containing a pinhole  108  positioned at its top focus  102 , act to suppress the second scattered light  116  from scattering site(s) on the bottom surface  126  of the transparent wafer  106 , sufficiently, to permit only first scattered beam  114  from top surface particles to be detected by scatter sensor  111 . The suppression of scattered light from the bottom surface  126  occurs because the source of the scattered light (the bottom surface) is far from the bottom focus  128  of the elliptical mirror  110  as shown in  FIG. 1 . The second scattered beam  116 , representing light scattered from the bottom surface  126 , is collected inside the elliptical mirror  110  and it is not focused at the top focus  102  of the elliptical mirror  110  but strikes the opaque surface  121  at a position away from the top focus  102  and, therefore, substantially no scattered light from scattering sites on the bottom surface  126  is allowed to pass through the pinhole which is placed at the top focus of the elliptical mirror  110 . Signals reaching scatter sensor  111  are substantially restricted to scattered light from the top surface  124  of the transparent wafer  106 . In accordance with another embodiment of the present invention, the amount of light received by scatter sensor  111  from the bottom surface  126  can be further reduced by adjusting the pinhole  108  diameter d to be substantially less than the thickness  130  of the transparent wafer  106 . 
   A substantial amount of the first scattered beam  114  passes through the pinhole  108  since the incoming beam  104  is focused at the bottom focus  128  of the elliptical mirror  110 . According to an embodiment, the depth of focus of elliptical mirror  110  can be determined by the diameter of pinhole  108 . In particular, for a pinhole of diameter “d” the depth of focus of the mirror will be roughly ±d/2. According to an embodiment of the present invention, a significant rejection of scattering site detection from the bottom surface  126  is accomplished by combining a pinhole  108  that is smaller (preferably much smaller) than the transparent wafer thickness  130  and an incoming beam  104  that is sharply focused. In an embodiment of the present invention, the incoming beam  104  is sharply focused by way of a high numerical aperture. In this manner, the reduction in bottom surface (or near bottom surface) scattering intensity is multiplied, due to the beam divergence and the limited depth of focus of the elliptical mirror  110 . 
     FIG. 2  illustrates the case of imaging defects on the bottom surface  226  of the transparent wafer  206  while not detecting defects on the top surface  224  of the transparent wafer  206 . The incoming optical beam  204  (from a laser or another optical source) is directed, by adjusting the position of the wafer  206  relative to the ellipsoid  210 , so that part of the incoming beam  204  passes through the wafer  206  and comes to a focus which is coincident with the bottom focus  228  of the elliptical mirror  210  which is now located at the bottom surface  226  of the wafer. Another portion, second scattered beam  216 , of incoming beam  204  reflects from the top surface  224  of transparent wafer  206  and is blocked by the opaque surface  221 . Yet another portion  220  of incoming optical beam  204  passes completely through the top  224  and bottom  226  surface of transparent surface  206 . A first portion  212  of incident beam  204  reflects from the top surface  224  and escapes through a hole in the side of the elliptical mirror  210 . 
   First scattered light  214  from the bottom surface  226  of transparent wafer  206  reflects from the elliptical mirror  210  and is directed to the top focus  202  of the mirror  210  where it passes through the pinhole  208  and is directed to a scattered light sensor  211 . The scattered light sensor  211  may be a photo multiplier tube, an avalanche photodiode or a PIN photodiode. Second scattered light beam  216 , from the top surface  224  of the wafer  206 , reflects from the elliptical mirror  210  in a manner that it does not pass through the pinhole  208 . The pinhole  208  is a circular or non-circular hole of diameter d in an opaque surface  221  that is placed in a plane perpendicular to the plane of the mirror and contains the top focus  202  of the elliptical mirror  210 . The smaller the diameter d of the pinhole  208 , the narrower (in the thickness  230  direction) is the depth over which scattered light will be detected. In the case of a pinhole which is approximately the same diameter as the focused beam, the thickness range over which scattered light is detected is roughly half the diameter of the pinhole. If, for example, the focused laser beam (as well as the pin hole) is 1 micron in diameter then the scattered light will be collected from a thickness range of +−5000 Angstroms about the bottom focal point of the ellipsoid. This can be estimated by assuming a pair of light rays  214  incident at 45° upon a pinhole of diameter d. These rays cross one another at the top focus  202  of the elliptical mirror  210  and they will expand to a diameter d when they have moved a distance ±d/2 from the top focal point  202 . This means that scattered light emanating from points, which are more than ±d/2 above or below the bottom focus  228  of the elliptical mirror, do not entirely pass through the pinhole and experience substantial attenuation. 
   Another embodiment of the present invention inspects both sides of a transparent wafer, with access to a single side at a time. That is, it is possible to locate optical body  100  or  200  above the top surface of a transparent wafer and to scan the top surface  124  for defects using the configuration shown in  FIG. 1  and to scan the bottom surface  226  using the configuration shown in  FIG. 2 . The inspection of the top surface may be done serially by first scanning the top surface  124  ( FIG. 1 ) at bottom focus  128  and then moving the bottom focus  228  down to the bottom surface  226  for scanning the bottom surface  226  ( FIG. 2 ). 
   In another embodiment, when using a sufficiently small pinhole, it is possible to image internal cross sectional planes that lay within the bulk of the transparent wafer without interference from either the top or bottom surfaces. This can be enhanced by using P polarized light and operating at an angle of incidence that is near the Brewster&#39;s angle for the particular material that is being examined. This embodiment allows a maximum amount of light to penetrate through the substrate and therefore a maximum amount of energy is available to image internal defects within the bulk of the transparent wafer. P scattered light also optimizes the amount of scattered light from small particles. 
     FIG. 3  illustrates a method for detecting particles or defects on the top, bottom or within the bulk of a transparent wafer or disk. First the position of the focus of the optical beam is adjusted  302  to be at the desired depth upon the transparent material. Then the position of the bottom focus of the elliptical mirror is adjusted  304  to be at the same position as the focus of the optical beam. The elliptical mirror receives  306  the scattered light from a region near the focus of the optical beam. The scattered light in a thickness region approximately ±d/2 is passed  308  through the pinhole of diameter d. A light sensitive device such as a PMT receives and detects  310  the scattered light. 
   Many technologically important materials are opaque in the visible light region (400 to 700 mn). However, many of these materials may be transparent at longer or shorter wavelengths. For example, Silicon is opaque to visible light but becomes transparent at wavelengths longer than 1.1 microns. Thus if Silicon is imaged with, for example 1.55 micron light it will be transparent and all the ideas discussed above may be applied to the inspection of Silicon wafers. Similar ideas may be applied to GaAs except even longer wavelengths would be required to make GaAs transparent. 
   While the invention has been particularly shown and described with reference to a preferred embodiment and various alternate embodiments, it will be understood by persons skilled in the relevant art that various changes in form and details can be made therein without departing from the spirit and scope of the invention.