Abstract:
An optical system for detecting defects on a wafer that includes a device for producing a beam and directing the beam onto the wafer surface, producing an illuminated spot on the wafer&#39;s surface. The system further includes a detector detecting light, and a mirrored assembly having together with the detector an axis of symmetry about a line perpendicular to the wafer surface. The assembly is configured to receive scattered light from the surface, where the scattered light including a first scattered light part being scattered from the pattern. The assembly is further configured to reflect and focus rotationally symmetrically about the axis of symmetry the scattered light to the detector. The system further includes a device operating with the detector for facilitating detection of a scattered light other than the specified scattered light due to pattern.

Description:
RELATED APPLICATION  
       [0001]     The present application is a Divisional of U.S. patent application Ser. No. 10/208,113, filed Jul. 29, 2002, entitled, “Process and Assembly For Non-Destructive Surface Inspection” This patent application is hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to the field of optical inspection. More specifically, the invention relates to the inspection of surfaces and in particular to detecting defects in semiconductor patterned wafers.  
       BACKGROUND OF THE INVENTION  
       [0003]     The detection of defects on the surface of semiconductor wafers due to imperfect production or the post-production adhesion process has received considerable attention in the art. Generally, wafers fall into two main categories, “unstructured” (or “unpatterned”) and “patterned”. A patterned wafer has circuit patterns (“dies”) imprinted on it, while an unstructured (unpatterned) wafer is still bare, i.e. with no circuits imprinted on it as yet.  
         [0004]     Generally speaking, numerous systems and methods have been developed to cope with the problem of defect detection and in particular, for the non-destructive inspection of silicon wafers. A prior art system known as the “Excite System” of Applied Materials includes a light beam source and an optical system that projects the beam onto the test object, as well as means for detecting the reflected and/or scattered light. There is an additional assembly for moving the test object in a coordinated translational and rotary movement, so that the light spot projected thereon scans the whole surface along a spiral path. The detected scattered light is analyzed in order to determine the sought defects.  
         [0005]     The development of processes enabling the manufacture of wafer surfaces with ever-finer structures, urged the development of inspection systems for the detection of ever more minute defects such as particle contamination, polishing scratches, variations in the thickness of coatings, roughness, crystal defects on and below the surface, etc. Insofar as unstructured wafers are concerned, they are subjected to a thorough searching examination for detecting said defects.  
         [0006]     In the chip manufacturing process, it is common to monitor each stage in order to recognize problems as early as possible and thus avoid undue waste. When unstructured wafers are compared between two process stages, the types and amount of defects at some stage can be determined. The inspected surface may be rough and metallized, and may therefore produce a great deal of scattered light, or, it may be a film-coated surface with a small amount of defects and produce scattered light. Thus, the inspecting instrument should preferably have a wide dynamic range of detection to permit defect and particle detection of a wide variety of surfaces.  
         [0007]     Laser scanners are particularly suitable for that purpose. Note that presently available laser scanners differ in the type of scanning they use, their optical configuration, and the manner in which the results are processed. For applications that require a high throughput and nearly 100% inspection of the whole wafer surface, two processes are mainly used. In the first, disclosed e.g. in U.S. Pat. No. 4,314,763 to Steigmeier &amp; Knop, the illuminating beam and the collecting optics are stationary, and the test object is scanned spirally by means of a coordinated translational and rotary movement of the object itself. In the second process disclosed, e.g. in U.S. Pat. No. 4,378,159 to Galbraith, a rotating or vibrating mirror moves the illuminating beam in one direction linearly back and forth across the wafer, while the wafer is simultaneously translated perpendicular thereto. In general, the first method is simpler and with homogenous accuracy, while the second is faster.  
         [0008]     Bearing all that in mind, attention is drawn to U.S. Pat. No. 6,271,916 to Marxer et al. Briefly speaking, the Marxer patent discloses an assembly for non-destructive surface inspections. The system according to the &#39; 916  patent will now be briefly described with reference to FIGS.  1 A-B. Thus, the apparatus according to the Marxer patent includes a light beam that is directed by a beam deflector  131  towards the wafer&#39;s surface  135 , preferably normal thereto. The wafer is moved by a rotation motor  145  and a translation motor  149  according to the technique disclosed in the &#39; 159  patent. A circumferential ellipsoidal mirrored surface  127  is placed around the wafer, with its axis coinciding with the surface normal, to collect scattered light from defects at the wafer surface at collection angles away from the surface normal. In some applications, a lens arrangement with its axis coinciding with the surface normal is also used to collect the light scattered by the surface and by any defects on it. The light scattered by the mirror and lenses may be directed to the same or different detectors. Preferably, light scattered by the surface within a first range of collection angles from the axis is detected by a first detector  121 , and light scattered by the surface within a second range of collection angles from the axis is detected by a second detector  125  (shown in  FIG. 1B  only). The two ranges of collection angles are different, with one detector optimized to detect scattering from large defects (mainly large particles) and the other detector optimized to detect light from small defects (particles). The content of the Marxer patent is incorporated herein by reference.  
         [0009]     The detectors according to the Marxer patent, detect practically only light scattered from defects, whereas reflected light (reflected from a well-polished surface) is out-guided in order not to interfere with the scattered light received by the detectors. This method of measuring diffused light from defects only is called “dark field”.  
         [0010]     The apparatus according to the Marxer patent offers a solution applicable, if at all, to the detection of defects on unstructured wafers. However, the specified apparatus of the Marxer patent is not applicable to the detection of defects on patterned wafers, because in the case of patterned wafers, the detectors do not only receive light scattered from defects, but also light scattered from the patterns. Considering that the intensity of the latter is much higher than that of the former, it would be very difficult and in fact practically infeasible to determine whether the received light originates from a defect or from a fault-free pattern.  
         [0011]     Die to die defect analysis is based upon a comparison (usually a on a pixel to pixel bases or even a sub-pixel to sub-pixel bases) of pixels originating from light scattered from the same spot on two distinct dies. Die to die comparison require that substantially the same illumination and collection conditions apply during the generation of the pixels. The Marxer patent does not enable die to die defect analysis as the wafer is rotated during the illumination of the wafer, and both the illumination and collection paths constantly change as result from the wafers rotation. The problem is especially acute when the wafers are patterned and when using dark field detectors to detect defects, as the dark field images are very dependent upon the direction of light scattered from the rotating pattern.  
         [0012]     Accordingly, there is a need in the art to provide an apparatus that performs defect detection of patterned wafers.  
         [0013]     There is another need in the art to provide an apparatus that performs defect detection of both patterned and unpatterned wafers.  
         [0014]     There is yet a further need to allow a compact optical inspection apparatus that enables die to die defect analysis.  
       SUMMARY OF THE INVENTION  
       [0015]     The invention provides for an optical system for detecting defects on a wafer that includes at least one pattern; the system comprising: 
        a source of light to produce a beam;     optics directing the beam along a path onto the wafer, producing an illuminated spot thereon;     at least one detector for detecting light; 
 
 an ellipsoidal mirrored surface, said mirrored surface and the at least one detector having an axis of symmetry about a line perpendicular to the wafer surface, said mirrored surface defining an input aperture positioned proximate to the wafer surface to receive scattered light therethrough from the surface; said mirrored surface further defining an exit aperture and being substantially rotationally symmetric about said axis of symmetry, so that the mirrored surface reflects and focuses rotationally symmetrically about said axis of symmetry light that passes through the input aperture to the at least one detector; 
    said exit aperture being located opposite to the input aperture; and     at least one filter located between said exit aperture and said at least one detector and being configured to pass to said at least one detector scattered light rays substantially other than scattered light part being scattered from at least one of said patterns.        
 
         [0021]     The invention enables die to die defect analysis by implementing at least one of the following measures: (i) illuminating the inspected object with a beam that is perpendicular to the surface of the inspected object, whereas the beam cross section is symmetrical and an array of detectors are positioned such as to collect scattered light beams ; (ii) using a dove prism; rotating the optical detectors array such as to compensate for the rotation of the wafer.  
         [0022]     The invention further provides for an optical system for detecting defects on a wafer that includes at least one pattern; the system comprising: 
        source of light to produce a beam;     optics directing the beam along a path onto the wafer surface, producing an illuminated spot thereon;     an array of detectors detecting light;     an ellipsoidal mirrored surface, said mirrored surface and the array of detectors having an axis of symmetry about a line perpendicular to the wafer, said mirrored surface defining an input aperture positioned proximate to the test surface to receive scattered light therethrough from the surface; said mirrored surface further defining an exit aperture and being substantially rotationally symmetric about said axis of symmetry, so that the mirrored surface reflects and focuses rotationally symmetrically about said axis of symmetry light that passes through the input aperture to the array of detectors;     said exit aperture being located between said array of detectors and said input aperture;     said array of detectors are adapted to detect scattered light substantially other than scattered light part being scattered from at least one of said patterns.        
 
         [0029]     Still further, the invention provides for an optical system for detecting defects on a wafer that includes at least one pattern; the system comprising: 
        a source of light to produce a beam;     optics directing the beam along a path onto the wafer, producing an illuminated spot thereon;     at least one detector for detecting light;     an ellipsoidal mirrored surface, said mirrored surface and the at least one detector having an axis of symmetry about a line perpendicular to the wafer surface, said mirrored surface defining an input aperture positioned proximate to the wafer surface to receive scattered light therethrough from the surface; said mirrored surface further defining an exit aperture and being substantially rotationally symmetric about said axis of symmetry, so that the mirrored surface reflects and focuses rotationally symmetrically about said axis of symmetry light that passes through said input aperture to the at least one detector;     said exit aperture being located opposite to the input aperture;     a Dove prism, having with the at least one detector an axis of symmetry about a line perpendicular to the wafer&#39;s surface and parallel to said Dove prism&#39;s base, said Dove prism being rotated about said axis of symmetry, so as to rotate light passing through said Dove prism at twice the angular velocity of said Dove prism in the opposite direction about said axis of symmetry; and     at least one filter located between said Dove prism and said at least one detector and being configured to pass to said at least one detector scattered light rays substantially other than scattered light part being scattered from at least one of said patterns.        
 
         [0037]     Yet further, an optical system for detecting defects on a wafer that includes at least one pattern; the system comprising: 
        a source of light to produce a beam;     optics directing the beam along a path onto the wafer, producing an illuminated spot thereon;     an array of detectors for detecting light;     an ellipsoidal mirrored surface, said mirrored surface and the array of detectors having an axis of symmetry about a line perpendicular to the wafer surface, said mirrored surface defining an input aperture positioned proximate to the wafer surface to receive scattered light therethrough from the surface; said mirrored surface further defining an exit aperture and being substantially rotationally symmetric about said axis of symmetry, so that said mirrored surface reflects and focuses rotationally symmetrically about said axis of symmetry light that passes through said input aperture to the array of detectors;     said exit aperture being located opposite to said input aperture;     a Dove prism, said Dove prism and the array of detectors having an axis of symmetry about a line perpendicular to the wafer surface and parallel to said Dove prism&#39;s base, said Dove prism being rotated about said axis of symmetry, so as to rotate light passing through said Dove prism at twice the angular velocity of said Dove prism in the opposite direction about said axis of symmetry; said Dove prism further being configured to pass to said array of detectors scattered light rays substantially other than scattered light part being scattered from at least one of said patterns.        
 
         [0044]     The invention provides for an optical system for detecting defects on a wafer that includes at least one pattem; the system comprising: 
        a source of light to produce a beam;     means for directing the beam along a path onto the wafer, producing an illuminated spot thereon;     at least one means for detecting light;     an ellipsoidal mirrored surface, said mirrored surface and the at least one detecting means having an axis of symmetry about a line perpendicular to the wafer surface, said mirrored surface defining an input aperture positioned proximate to the wafer surface to receive scattered light therethrough from the surface; said mirrored surface further defining an exit aperture and being substantially rotationally symmetric about said axis of symmetry, so that the mirrored surface reflects and focuses rotationally symmetrically about said axis of symmetry light that passes through the input aperture to the at least one detecting means; said exit aperture being located opposite to the input aperture; and     at least one filter located between said exit aperture and said at least one detecting means andbeing configured to pass to said at least one detecting means scattered light rays substantially other than scattered light part being scattered from at least one of said patterns.        
 
         [0050]     The invention further provides for an optical system for detecting defects on a wafer that includes at least one pattern; the system comprising:  
         [0000]     a source of light to produce a beam;  
         [0000]    
       
         
           
              means for directing the beam along a path onto the wafer surface, producing an illuminated spot thereon;  
              an array of detecting means for detecting light;  
              an ellipsoidal mirrored surface, said mirrored surface and the array of detecting means having an axis of symmetry about a line perpendicular to the wafer, said mirrored surface defining an input aperture positioned proximate to the test surface to receive scattered light therethrough from the surface; said mirrored surface further defining an exit aperture and being substantially rotationally symmetric about said axis of symmetry, so that the mirrored surface reflects and focuses rotationally symmetrically about said axis of symmetry light that passes through the input aperture to the array of detecting means;  
              said exit aperture being located between said array of detecting means and said input aperture;  
              said array of detecting means are adapted to detect scattered light substantially other than scattered light part being scattered from at least one of said patterns.  
              Still further, the invention provides for an optical system for detecting defects on a wafer, comprising:  
              a device for producing a beam and directing the beam onto the wafer surface, producing an illuminated spot thereon;  
              at least one detector detecting light;  
              a mirrored assembly having together with the at least one detector an axis of symmetry about a line perpendicular to the wafer surface, said assembly is configured to receive scattered light from the surface; said assembly further configured to reflect and focus rotationally symmetrically about said axis of symmetry the scattered light to the at least one detector; and  
              a device associated with said at least one detector for facilitating detection of a scattered light substantially other than scattered light part being scattered from at least one of said patterns.  
           
         
       
     
         [0061]     Yet further, the invention provides for an optical system for detecting defects on a wafer, comprising: 
        a device for producing a beam and directing the beam onto the wafer surface, producing an illuminated spot thereon;     at least one detector detecting light;     a mirrored assembly configured to receive scattered light from the surface; said assembly further configured to reflect the scattered light to the at least one detector; and     a device associated with said at least one detector for facilitating detection of a scattered light substantially other than scattered light part being scattered from at least one of said patterns.        
 
         [0066]     The invention provides for an optical method for detecting defects on a wafer that includes at least one pattern; the method comprising: 
        providing a beam of light;     directing the beam along a path onto the wafer, producing an illuminated spot thereon;     positioning an ellipsoidal mirrored surface and at least one detector so that they have an axis of symmetry about a line perpendicular to the wafer surface, said mirrored surface defining an input aperture positioned proximate to the wafer surface to receive scattered light therethrough from the surface; said mirrored surface further defining an exit aperture and being substantially rotationally symmetric about said axis of symmetry, so that the mirrored surface reflects and focuses rotationally symmetrically about said axis of symmetry light that passes through the input aperture to the at least one detector; said exit aperture being located opposite to the input aperture; and     locating at least one filter between said exit aperture and said at least one detector, configuring the at least one filter to pass to said at least one detector scattered light rays substantially other than scattered light part being scattered from at least one of said patterns.        
 
         [0071]     Still further, the invention provides for An optical method for detecting defects on a wafer that includes at least one pattern; the method comprising: providing a beam of light; 
        directing the beam along a path onto the wafer surface, producing an illuminated spot thereon;     positioning an ellipsoidal mirrored surface and an array of detectors so that they have an axis of symmetry about a line perpendicular to the wafer, said mirrored surface defining an input aperture positioned proximate to the test surface to receive scattered light therethrough from the surface; said mirrored surface further defining an exit aperture and being substantially rotationally symmetric about said axis of symmetry, so that the mirrored surface reflects and focuses rotationally symmetrically about said axis of symmetry light that passes through the input aperture to the array of detectors; said exit aperture being located between said array of detectors and said input aperture;     adapting said array of detectors to detect scattered light substantially other than scattered light part being scattered from at least one of said patterns.        
 
         [0075]     Yet further, the invention provides for an optical method for detecting defects on a wafer that includes at least one pattern; the method comprising: 
        providing a beam of light;     directing the beam along a path onto the wafer, producing an illuminated spot thereon;     positioning an ellipsoidal mirrored surface and at least one detector so that they have an axis of symmetry about a line perpendicular to the wafer surface, said mirrored surface defining an input aperture positioned proximate to the wafer surface to receive scattered light therethrough from the surface; said mirrored surface fiurther defining an exit aperture and being substantially rotationally symmetric about said axis of symmetry, so that the mirrored surface reflects and focuses rotationally symmetrically about said axis of symmetry light that passes through said input aperture to the at least one detector; said exit aperture being located opposite to the input aperture;     positioning a Dove prism, so as to have with the at least one detector an axis of symmetry about a line perpendicular to the wafer&#39;s surface and parallel to said Dove prism&#39;s base; said Dove prism is rotated about said axis of symmetry, so as to rotate light passing through said Dove prism at twice the angular velocity of said Dove prism in the opposite direction about said axis of symmetry; and     locating at least one filter between said Dove prism and said at least one detector and configuring the filter to pass to said at least one detector scattered light rays substantially other than scattered light part being scattered from at least one of said patterns.        
 
         [0081]     The invention provides for an optical method for detecting defects on a wafer that includes at least one pattern; the method comprising: 
        providing a beam of light;     directing the beam along a path onto the wafer, producing an illuminated spot thereon;     positioning an ellipsoidal mirrored surface and an array of detectors so that they have an axis of symmetry about a line perpendicular to the wafer surface, said mirrored surface defining an input aperture positioned proximate to the wafer surface to receive scattered light therethrough from the surface; said mirrored surface further defining an exit aperture and being substantially rotationally symmetric about said axis of symmetry, so that said mirrored surface reflects and focuses rotationally symmetrically about said axis of symmetry light that passes through said input aperture to the array of detectors; said exit aperture being located opposite to said input aperture;     positioning a Dove prism so as to have with the array of detectors an axis of symmetry about a line perpendicular to the wafer surface and parallel to said Dove prism&#39;s base; said Dove prism is rotated about said axis of symmetry, so as to rotate light passing through said Dove prism at twice the angular velocity of said Dove prism in the opposite direction about said axis of symmetry; said Dove prism is further being configured to pass to said array of detectors scattered light rays substantially other than scattered light part being scattered from at least one of said patterns.        
 
         [0086]     The invention further provides for an optical method for detecting defects on a wafer, comprising: 
        providing a device for producing a beam and directing the beam onto the wafer surface so as to produce illuminated spot thereon;     positioning a mirrored assembly and at least one detector so that they have an axis of symmetry about a line perpendicular to the wafer surface; configuring said assembly to receive scattered light from the surface and further configuring said assembly to reflect and focus rotationally symmetrically about said axis of symmetry the scattered light to the at least one detector; and     positioning a device associated with said at least one detector for facilitating detection of a scattered light substantially other than scattered light part being scattered from at least one of said patterns.        
 
         [0090]     Yet further, the invention provides for an optical method for detecting defects on a wafer, comprising: 
        providing a device for producing a beam and directing the beam onto the wafer surface so as to produce an illuminated spot thereon;     positioning a mirrored assembly configured to receive scattered light from the surface and further configuring said assembly to reflect the scattered light to the at least one detector; and     positioning a device associated with said at least one detector for facilitating detection of a scattered light substantially other than scattered light part being scattered from at least one of said patterns.        
 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0094]     In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the acompanying drawings, in which:  
         [0095]      FIG. 1A -B illustrate schematically two embodiments of an apparatus according to the Marxer patent;  
         [0096]      FIG. 2A  shows a perspective view of an apparatus according to one embodiment of the present invention;  
         [0097]      FIG. 2B  shows a perspective view of an apparatus according to another embodiment of the present invention;  
         [0098]      FIG. 2C  shows a perspective view of an apparatus according to still another embodiment of the present invention;  
         [0099]      FIG. 3A  shows a perspective view of a filter placed in the apparatus according to an embodiment of the present invention;  
         [0100]     FIGS.  3 B-C describe the rotation of the filter synchronized with the wafer&#39;s rotation, in accordance with an embodiment of the invention;  
         [0101]     FIGS.  4 A-B show a side view of a closed and open MEMS (Micro-Electro-Mechanical System, in accordance with an embodiment of the invention;  
         [0102]      FIG. 4C  shows a plan view of the MEMS illustrated in  FIGS. 5A and 5B ;  
         [0103]      FIG. 4D  shows a filter comprised of a 2D matrix of MEMS, in accordance with an embodiment of the invention;  
         [0104]     FIGS.  5 A-B illustrate two stages (pass/fail) of a liquid crystal based filter in accordance with an embodiment of the invention;  
         [0105]      FIG. 5C  shows a filter that is composed of 2D matrix liquid crystal cells, in accordance with an embodiment of the invention;  
         [0106]      FIG. 6A  shows a ring-shaped array of detectors, in accordance with an embodiment of the invention;  
         [0107]      FIG. 6B  shows concentric ring-shaped arrays of detectors, in accordance with an embodiment of the invention;  
         [0108]      FIG. 7A  shows a perspective view of an array of detectors made of CCDs, in accordance with an embodiment of the invention;  
         [0109]     FIGS.  7 B-C show respective opening and closing of CCDs according to wafer rotation, in accordance with the embodiment of  FIG. 8A ;  
         [0110]      FIG. 8A  illustrates the principle of operation of a Dove prism;  
         [0111]      FIG. 8B  further exemplifies the rotation of an image due to rotation of a Dove prism; and  
         [0112]      FIG. 8C  shows a perspective view of an apparatus according to still another embodiment of the present invention, using a Dove prism between mirror to detector.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0113]     It should be noted that in the context of the invention, the term “defect” should be construed in a broad manner including but not limited to particle contamination, polishing scratches, variations in the thickness of coatings, roughness, crystal defects on and below the surface etc.  
         [0114]     A beam of light that impinges on the surface of a patterned wafer produces a reflected beam and multiple scattered rays due to pattern and due to (possible) defects. The distribution of scattered rays due to pattern is distributed substantially different than the distribution of scattered ray due to defects. Thus, knowing in advance the distribution of scattered rays due to pattern, enables one to locate a set of detectors at places where the distribution of scattered rays due to pattern is zero, or at least minimal, or alternatively block the light rays at places where the distribution of rays due to pattern is significant, and by this to detect mainly scattered rays due to defects. This method can be used to verify the presence of defects on a patterned wafer, as will be explained in detail below.  
         [0115]      FIG. 2A  shows apparatus  101  according to an embodiment of the present invention. The apparatus differs from that described in U.S. Pat. No. 6,271,916 B1, in that it is operable to detect defects of patterned wafers. Apparatus  200  includes filter  310  that is disposed between an ellipsoidal mirror  127  and a detector  121 . The added filter is designed to block the path of scattered light due to the pattern, but simultaneously let the scattered light due to a defect pass through, as will be explained in greater detail below.  
         [0116]      FIG. 2B  shows apparatus  102  in accordance with another embodiment of the invention. Apparatus  102  is similar to apparatus  101 , but has multiple detectors, such as detector array  320 , instead of filter  310 .  
         [0117]      FIG. 2C  shows apparatus  103  in accordance with a further embodiment of the invention. Apparatus  103  is similar to apparatus  101  but has additional multiple detectors, such as detector array  320  located upstream in the paths of scattered and reflected light beams in relation to filter  310 . The addition of these detectors enables to geometrically select the pattern of scattered light-rays to be detected, which improves the system&#39;s performance.  
         [0118]     Note that although the light beam that impinges the wafer surface in the examples of  FIGS. 2A and 2B  is perpendicular to the surface, the invention is by no means bound by these specific embodiments, as exemplified in  FIG. 2C . An exemplary embodiment where the light impinges the surface in a non-perpendicular fashion is described with reference to  FIG. 8C -D, below.  
         [0119]     Further note that whereas for the convenience of explanation the description below concerns mainly a filter, it likewise applies to an array of filters.  
         [0120]     It also should be noted that the invention is by no means bound by this specific embodiment. Thus, for example, in accordance with a modified embodiment, any of the previous embodiments can be modified, e.g. to include a first optical means that collimate the scattered light to be filtered and second optical means to focus the filtered beam before impinging on the detector. The first optical means can be used also for matching the diameter of the collimated beam to the diameter of the filter  310  or to the diameter of the detector array  320 . Both optical means can be used also for fine-tuning the solid angle of the beam of light impinging on the filter and/or the detector. In still another modified embodiment a lens assembly is disposed between the input aperture and the exit aperture of ellipsoidal mirrored surface  127  to collect light reflected from the wafer surface and passing through the mirrored surface. The reflected light is then guided away from the filter to be blocked or used for further detection.  
         [0121]     Generally speaking, when the wafer rotates, different dies on the wafer are exposed to the illumination spot for inspection purposes. Since all dies in the wafer have the same orientation, it readily arises that as the wafer rotates, the inspected die&#39;s pattern is oriented at a different angle for each rotation. Accordingly, by the embodiments of  FIGS. 2A-2C , whichever the case may be, the filter  310  and/or detectors  320  should be rotated or reconfigured in synchronization with that of the wafer rotation such that the filter will substantially block the scattered light due to the pattern and, by the same token, the detectors will be substantially blocked from detection of the scattered light due to the pattern. In contrast, the filter should substantially pass scattered light due to defect(s) and by the same token, the detectors should substantially detect light due to defect(s). By “substantially” it is meant that not all scattered light due to defect(s) is passed or detected, which the case may be. To this end, a controller  150  is utilized, as will be explained in greater detail below.  
         [0122]     There are several ways to realize the above described process, as will now be explained with reference to  FIG. 3  and onwards. As shown in  FIG. 3A , apparatus  101  has filter  310  that includes disk  401 , which is opaque to light. In the disk  401  there are apertures  403  at pre-defined locations such that scattered light due to defects can pass through said filter only through said apertures. A controller  150  that is coupled to both the disk  401  and rotation motor  143  is configured to rotate the disk about the axis of symmetry SR in a synchronized fashion with said motor, so as to let the light scattered from the defects pass through said apertures and be detected by the detector  121 . The scattered light (or major portion thereof) due to the pattern, impinges on the opaque sectors  401  and is blocked thereby, and consequently, will not reach detector  121  and obviously will not be detected. Thus, the filter functions as a rigid mask to substantially block all scattered light rays due to the pattern in accordance with the (same) rotational movement of all the detectors.  
         [0123]     It should be noted that, as known per se, the shape of the filter (e.g. in the form of disk), and in particular the pattern of apertures such as  403 , is tailored to fit the pattern of the surface. Such filters are known in the art. Such a disk may be either designed e.g. in accordance with actual measurements of the distribution of light reflection due to pattern, or as a consequence of a mathematical model describing the reflection and scattering pattern from a specific wafer.  
         [0124]     Further note that a filter bank can be prepared in advance for a variety of requirements. A filter assembly composed of several filters (e.g. in the form of a large disk that contains several filters along its perimeter) can be used as filter  403 . For each type of patterned wafer, the most suitable filter on the filter assembly is chosen in accordance with actual measurements of the distribution of light reflection due to pattern received at the detector after being reflected by a each filter available on the assembly and selecting the most appropriate one.  
         [0125]     The filter ensures that the scattered light rays that reach the detectors are mainly or wholly due to defects. The rotation of the filter is exemplified in  FIG. 3B -C.  
         [0126]     Those versed in the art will readily appreciate that the invention is not bound by the use of a disk with discrete apertures and particularly not to the disk described in FIGS.  3 A-C.  
         [0127]     Another non-limiting realization of a filter is the Micro-Electro-Mechanical System (MEMS) 2D array technique shown in  FIG. 4A -D. The MEMS array-of-shutters functions exactly as a filter described above with reference to FIGS.  3 A-C, except for the fact that the rotation is done electronically. Thus,  FIG. 4A  schematically describes the structure of a rectangular MEMS shutter. It is noted that other shaped MEMS arrays, such as circular MEMS arrays may be utilized. MEMS arrays are known in the art.  FIG. 4A -D illustrate a typical MEMS array. The etching of a thick silicon wafer bedding  500  produces a thin sheet of silicon  505  that is partially surrounded by a narrow spacing  507  and is also connected to a thick sheet of silicon  503 . The narrow spacing  507  is generated by etching the whole silicon thick layer. Therefore, the thin sheet  505  is able to bend to some degree. The addition of electrodes  509  enables to realize the bending of the thin silicon sheet ( FIG. 4B ) or aligning it back ( FIG. 4A ), thus giving rise to an opening or closing of the shutter. The MEMS technique enables to produce a 2D array of, say, hundreds of shutters, a number large enough to make an effective filter for the purpose of the present invention, as shown schematically in  FIG. 4D .  
         [0128]     In order to block scattered light due to a die pattern, some of the shutters should be closed in a pattern that fits the pattern of said scattered light, i.e. scattered light due to the pattern should be blocked (by impinging on closed shutters) and scattered light due to defects should pass through open shutters, similar to the configuration described above, with reference to  FIG. 3A . To this end, a controller of the kind described above should be employed to synchronize between the wafer rotation and the opening/closing of the MEMS shutters.  
         [0129]     Those versed in the art will readily appreciate that the invention is not bound by the use of an array of shutters and particularly not to the MEMS array described in FIGS.  4 A-D.  
         [0130]     Another non-limiting realization of a filter is the Liquid Crystal Display (LCD) technique, presented in FIGS.  5 A-C. As is generally known per se, LCD unit (or cell) is a device having a first polarizer and a second polarizer, both defining a space in which a liquid crystal is placed. A liquid crystal is fluid like a regular liquid but is anisotropic in its optical and electromagnetic characteristics like a solid, due to the high orientational order of the liquid crystal molecules ( 620  in  FIG. 5A ).  
         [0131]     When plane-polarized light passes through a liquid crystal, the molecules of the liquid crystal rotate the plane of polarization of the light. Light that passes through the first polarizer  640  is polarized. The polarized light passes then through the liquid crystal, which rotates the plane of polarization of the passing light. The second polarizer  660  is placed at the exit of the liquid crystal. The orientation of the second polarizer is chosen to be parallel to the polarization of the light emanating from the liquid crystal (e.g. perpendicular to first polarizer, but in no case parallel to it). Thus, the liquid crystal guides the polarized light from the first polarizer so that the light may be transmitted through the second polarizer.  
         [0132]     When an external voltage  610  is applied across a liquid crystal cell, the liquid crystal molecules ( 630  in  FIG. 5B ) are aligned in parallel to the electric field that is induced by the external voltage, and cannot rotate the plane of polarization of the passing light anymore. Thus, light cannot get out of the device any more. Therefore, applying a voltage on the LCD based device  601  in  FIG. 5B  blocks the light in analogy to the finctioning of the MEMS device. The LCD 2D array in  FIG. 5C  functions similarly to the 2D MEMS array explained above, where voltage activated (open) MEMS is functionally analog to a non-activated (open) LCD cell  600  and close MEMS is functionally analog to a voltage activated (close) LCD cell  601 . The device uses a controller of the kind specified above to synchronize between the opening/closing of LCD shutters and the wafer rotation.  
         [0133]     Those versed in the art will readily appreciate that the invention is not bound by the use of an array of shutters and particularly not to the liquid crystal array described in FIGS.  5 A-C.  
         [0134]     Note that the blocking of scattered light due to the die&#39;s pattern can be realized also by using an array of detectors. The array of detectors is adapted to detect scattered light substantially other than said scattered light due to a pattern. By one embodiment, this is realized in a way that the detectors in the array are switched on or off via a controller in a synchronized manner to the rotation of the wafer as in the case of a filter described with reference to  FIG. 2A  above. By way of another example, this may be realized by switching on all the detectors but reading under the control of the controller only data indicative of scattered light substantially other than said scattered light due to pattern  
         [0135]     Note that each detector has its own light collection zone. The light collection zones of different detectors may vary in shape, in size and/or in their direction. The light collection zones of neighboring detectors preferably partially overlap, so as to ensure coverage of the whole detection area.  
         [0136]     There may be many realizations that utilize an array of detectors. There follows a description of two non-limiting embodiments.  
         [0137]     In accordance with a first realization described with reference to  FIG. 6A , detector array  320  includes a plurality of detectors collectively denoted  700  that are arranged in a ring shape. Such a device may include e.g. several tens of detectors, such as detectors  701  (e.g. Photo-multiplier Tubes (PMTs), Photodiodes, Avalanche Photodiodes), which is enough for obtaining considerable (but still rough) sensitivity to the rotation of dies.  
         [0138]     Detector array  320  may also include multiple detectors arranged as a few concentric rings, as illustrated at  FIG. 6B . All the ringed arrays of detectors are concentrically placed at one plane and oriented towards the same location. This configuration enables to add more detectors and improves the configuration&#39;s sensitivity to the angular orientation of the die.  
         [0139]     Those versed in the art will readily appreciate that the invention is not bound by the use of ringed arrays of detectors and particularly not to the array of detectors described in  FIGS. 6A  or  6 B.  
         [0140]     Another realization is shown in FIGS.  7 A-C.  FIG. 7A  illustrates apparatus  102  in which detector array  320  includes a CCD array. A CCD array  800  is an array of light-sensitive elements  802 , which are, in fact, some small electronic capacitors that are charged by the electrons that are generated by incident light. The array may be implemented by at least one CCD chip, each CCD chip including multiple CCD detecting elements. Common CCD chips are composed of a large number of detecting elements, referred to also as pixels (e.g. 192*165, 512*512, 1024*1024 or more). Thus, the use of a CCD array is advantageous as compared to an array of regular detectors, in that the number of detectors (i.e. cells) is enormously higher than in the former realization. This makes the CCD array much more accurate and sensitive to small angle rotations.  
         [0141]     The main disadvantage of using a CCD array is the huge data rate delivered as an output of the CCD. The sampling rate of dies on a wafer is very high, typically, although not necessarily, about 10 7  samples/sec. Thus, the data rate that should be delivered from a CCD is about N*10 7  pixels/sec, where N is the number of CCD elements. Since a typical CCD has about 10 4 -10 6  pixels, the expected output data rate is in the range of 10 11 -10 13  pixels/sec, which is well beyond the present technology. An example for a fast CCD array is the PB-MV40 Megapixel CMOS Image Sensor of Photobit Company, which is capable of a digital output of almost 10 9  pixels/sec per second, at most one percent of the expected rate.  
         [0142]     A non-limiting solution to the problem of the data-processing bottleneck is by reading only a partial set of elements at each sampling (e.g.  802 , not  804 ), since, anyway, not all of them are required for collecting scattered light from defects. Note that a CCD array composed of a large number of CCD chips, each chip having its own light collection zone, can be partitioned so as to allow such a selection, by avoiding data collection from chips that get scattered light due to pattern. Still, the amount of information is huge, rendering the data processing relatively complicated.  
         [0143]     Those versed in the art will readily appreciate that the invention is not bound by the use of a CCD array and particularly not to the CCD array described in FIGS.  7 A-C.  
         [0144]     Reverting now to  FIG. 2C , it is possible to use both a filter and an array of detectors. This combination adds a degree of freedom to geometrically select the pattern of scattered light-rays to be detected. This choice can improve the system&#39;s performance.  
         [0145]     Another way to realize the apparatus according to the present invention, is by rotating the beam of scattered light at the outlet of the mirror  127 , instead of rotating a filter or detector-array for the same purpose. This can be realized by using a Dove prism, whose principle of operation is schematically shown in  FIG. 8A . A Dove prism is a prism whose triangular head is truncated. When light rays enter an incidental wall  901  of Dove prism  900 , they get out of the other wall  902  in a reversed order. When the Dove prism  900  is rotated around an axis DP parallel to its base, as is shown in  FIG. 8B , the entering image is rotated too at twice the angular velocity of the prism. This is illustrated in  FIG. 8B , where the image  910  is rotated by 180° with reference to the object  905 , as compared to a 90° rotation of Dove prism  900 . Note that while rotating Dove prism  900  the field of view (e.g. an image  910  of a die on the wafer) is rotated too, but the latter does not change its location within the wafer&#39;s surface. Thus, using Dove prism  900  obviates the need to rotate the filter in a controlled fashion or to control the detector array as discussed above.  
         [0146]     The use of a dove prism  900  allows performing die to die defect analysis, as the dove prism rotation compensates for the wafers rotation. In other words, the dove prism provides substantially the same illumination and collection conditions, regardless the rotation of the wafer. An image of a die can be stored to be later compared to an image of another die or a to a golden die.  
         [0147]      FIGS. 8C and 8D  illustrate apparatus  104 , in accordance with a preferred embodiment of the invention. Apparatus  104  differs from apparatus  101  of  FIG. 2A  by setting Dove prism  900  disposed between the exit aperture of ellipsoidal mirror  127  and filter  310 . In addition, lens assembly  930  is set for focusing the light beam onto the wafer surface and lens assembly  940  for collimating the reflected beam at the outlet of ellipsoidal mirror  127 . Lens assembly  950  is set for collimating the light rays scattered from the wafer surface due to pattern and defects, while lens assembly  960  is set to focus the light at the outlet of filter  320  on detector  121 . Note that lens assembly  950  is needed only when filter  310  is used for apparatus  104 , but not in the alternative where detector array  320  is used. Note that controller  150  is required for rotating Dove prism  900  in a controlled fashion with the wafer rotation to compensate for the rotation of wafer pattern, as explained above.  
         [0148]     Thus, according to one embodiment of the present invention, schematically illustrated in  FIGS. 8C and 8D , Dove prism  900  is rotated at half the angular velocity of the wafer rotation in the opposite direction. The image at the exit of the Dove prism is a static image of a die, since the prism rotation compensates for the wafer rotation. The filter e.g.  310  or detectors array e.g.  320  as described above are placed at the exit of the Dove prism.  
         [0149]      FIG. 8C  describes the path of the bright-field light beam in the system. A beam of light is guided by mirror  920  to lens  320  that focuses the beam. The beam is guided through Dove prism  900  and the ellipsoidal mirror  127  onto the wafer surface. Lens  930  may be located also between Dove prism  900  and ellipsoidal mirror  127 , on the condition that it has a ring shape, to allow the path of scattered rays upstream. The reflected beam is guided back through ellipsoidal mirror  127  to lens  940  that collimates the beam. The collimated beam passes through Dove prism  900  and is guided away by mirror  970  to be blocked or for further detection for other applications.  
         [0150]      FIG. 8D  describes the dark-field light beam in the system, showing the path through the optical system of the scattered rays due to pattern and defects. Lens assembly  950  and  960  form a relay assembly, which is intended to image the plane proximate to the wafer surface at a remote plane where the detector or detector array are located. Note that the detector (detectors array) are disposed relatively away from the surface (requiring thus the image at the remote plane) due to the relatively large size of the Dove prism. Relay assembly  950  and  960  enable to detect the image as if it were closer to the wafer. Lens assembly further matches the diameter of the collimated beam to the diameter of filter  310 . The scattered light rays are collimated by lens assembly  950  to be guided through Dove prism  900 . At the outlet of the Dove prism the collimated beam passes through filter  310  and then is focused by lens assembly  960  to impinge onto detector  121 .  
         [0151]     Note that other embodiments of Dove Prism are applicable. For example, instead of using filter  310  and detector  121 , detectors array  320  is used. In still another embodiment of the present invention, filter  310  and detectors array  320  are used. The latter modifications are substantially similar to those described with reference  101 - 103  in FIGS.  2 A-C.  
         [0152]     Further note that the embodiment of apparatus  104  has two advantages over the embodiments of apparatuses  101 - 103  illustrated with reference to FIGS.  2 A-C. First, the image at the outlet of the Dove prism is static independent of the die&#39;s orientation. This means that the filter or the detector array gets practically the same distribution of light rays for each die. On the contrary, for the embodiments  101 - 103  the filter should be rotated in a controlled fashion (or the detector array operated in a controlled fashion) so as to match the image rotation. Thus, in the embodiments  101 - 103  unavoidable errors occur due to the limited resolution of the filter or of the detector array. The second advantage is that apparatus  104  is better adapted to use asymmetric light source, i.e. light source directed at a first angle in relation to the normal to the wafer surface (where the normal constitutes symmetry axis SR). For all the embodiments of FIGS.  2 A-C, the use of an asymmetric light source necessarily entails the rotation of the light source in a synchronized fashion with the wafer rotation, thus maintaining the same orientation with the pattern of each die. Otherwise the filter or detector array would receive a totally different distribution of scattered light due to the pattern for each die. In contrast, in apparatus  104 , as was explained above, the resulting image at the filter or detector array is static due to the Dove prism, and therefore there is no need to rotate the light source.  
         [0153]     Those versed in the art will readily appreciate that the invention is not bound by the use of a rotating prism and particularly not to the Dove prism described in FIGS.  8 A-C.  
         [0154]     The invention has been described with a certain degree of particularity. Those versed in the art will readily appreciate that the invention is not bound by the particular configurations described with reference to FIGS.  2  to  8  above or by the specific apparatus disclosed in the Marxer patent.