Inspection system for array of microcircuit dies having redundant circuit patterns

Abstract

An inspection system (10, 100) employs a Fourier transform lens (34, 120) and an inverse Fourier transform lens (54, 142) positioned along an optic axis (48, 144) to produce from an illuminated area of a patterned specimen wafer (12) a spatial frequency spectrum whose frequency components can be selectively filtered to produce an image pattern of defects in the illuminated area of the wafer. Depending on the optical component configuration of the inspection system, the filtering can be accomplished by a spatial filter of either the transmissive (50) or reflective (102) type. The lenses collect light diffracted by a wafer die (14) aligned with the optic axis and light diffracted by other wafer dies proximately located to such die. The inspection system is useful for inspecting only dies having many redundant circuit patterns. The filtered image strikes the surface of a two-dimensional photodetector array (58) which detects the presence of light corresponding to defects in only the illuminated on-axis wafer die. Inspection of all possible defects in the portions of the wafer surface having many redundant circuit patterns is accomplished by mounting the wafer onto a two-dimensional translation stage and moving the stage (40) so that the illuminated area continuously scans across the wafer surface from die to die until the desired portions of the wafer surface have been illuminated. The use of a time delay integration technique permits continuous stage movement and inspection of the wafer surface in a raster scan fashion.

Description

TECHNICAL FIELD

The present invention relates to inspection systems for use in the manufacture of microcircuits and, in particular, to a real-time defect inspection system for use in the manufacture of microcircuits of the type that includes an array of dies each having many redundant circuit patterns.

BACKGROUND OF THE INVENTION

Two exemplary and very similar inspection systems for pattern defects in photomasks employed in the large-scale manufacture of semiconductor devices and integrated circuits are described in U.S. Pat. Nos. 4,000,949 of Watkins and 3,614,232 of Mathisen. The systems of Watkins and Mathisen contemplate the simultaneous inspection of all of the dies on a photomask which contains a regular array of normally identical dies to detect the presence of nonperiodic defects, i.e., defects in one die not identically repeated in the remaining dies of the array.

This task is accomplished by illuminating simultaneously all of the dies of a specimen photomask with collimated coherent light emanating from a laser to develop a composite diffraction pattern whose spatial distribution is the combination of two components. The first component is the interference pattern of the array of dies, and the second component is the interference pattern of a single die of the array. The first and second components are sometimes called an inter-die interference pattern and an intra-die interference pattern, respectively. The light transmitted by the photomask strikes a double-convex lens which distributes the light on a spatial filter positioned a distance equal to one focal length behind the lens.

The spatial filter comprises a two-dimensional Fourier transform pattern of a known error-free reference photomask against which the specimen photomask is compared. The filter is opaque in the areas corresponding to spatial frequency components of the error-free Fourier transform pattern and is transparent in areas not included in the error-free Fourier transform pattern. Neither the Watkins patent nor the Mathisen patent specifies the design parameters of the lens. The Mathisen patent states only that the lens is of suitable numerical aperture and magnification power to cover the area of the specimen photomask.

The spatial frequency components corresponding to the defects in the specimen photomask are largely transmitted through the spatial filter and can be processed in either one of two ways. In the Watkins system, the light transmitted through the spatial filter strikes another double-convex lens that is properly positioned to define an image of the specimen photomask, absent any information blocked by the spatial filter. The imaging light not blocked by the spatial filter appears in locations that represent the position in the specimen photomask where defects are present. In the Mathisen system, the light transmitted through the spatial filter is sensed by a photodetector that produces an output signal which activates a "no-go" alarm.

The Watkins and Mathisen patents imply that systems of the type they describe require both inter- and intra-die interference pattern information to determine the presence of defects in the specimen pattern. The inter-die interference pattern information is of particular concern because it consists of very closely spaced light spots that are extremely difficult to resolve by a Fourier transform lens. The realization of such a lens is further complicated for inspection systems that use an inverse Fourier transform lens to form an image of the specimen pattern from the Fourier transform light pattern. The reason is that the design of each of the lenses is compromised to accomplish an overall system design that accomplishes both the Fourier transform pattern and image forming functions. It is, therefore, exceedingly difficult to obtain from such a system design the resolution required to acquire inter-die interference pattern information. The above lens design problem is encountered in systems of the type that simultaneously inspects the entire area of each of the dies of a specimen photomask array and, as a consequence, renders such systems unreliable and impracticable for commercial use.

SUMMARY OF THE INVENTION

An object of the present invention is, therefore, to provide a reliable defect inspection system for use in the manufacture of microcircuits.

Another object of this invention is to provide such a system that applies the techniques of Fourier optics but does not contemplate the use of inter-die interference pattern information to determine the presence of defects in the manufacture of microcircuits of the type that comprises an array of normally identical dies.

A further object of this invention is to provide such a system that is capable of developing from a microcircuit pattern an essentially aberration-free Fourier transform light pattern from which an accurate image corresponding to defects in the microcircuit pattern can be formed.

Still another object of this invention is to provide an inspection method that uses intra-die interference pattern information to determine the presence of defects in a microcircuit array pattern of normally identical dies.

The present invention relates to a method and system for use in the manufacture of microcircuits and is described herein by way of example only with reference to a real-time inspection system for defects in surfaces of semiconductor wafers of the type that includes an array of circuit dies of which each has many redundant circuit patterns. Such semiconductor wafers include, for example, random access and read only memory devices and digital multipliers.

Two preferred embodiments of the inspection system employ a Fourier transform lens and an inverse Fourier transform lens positioned along an optic axis to produce from an illuminated area of a patterned specimen wafer a spatial frequency spectrum whose frequency components can be selectively filtered to produce an image patter of defects in the illuminated area of the wafer. The lenses collect light diffracted by a wafer die aligned with the optic axis and light diffracted by other wafer dies proximally located to such die, rather than light diffracted by the entire wafer. This restriction limits the applicability of the inspection system to dies having many redundant circuit patterns but permits the use of lenses that introduce off-axis aberrations that would otherwise alter the character of the Fourier transform pattern and the filtered defect image.

Such lenses are relatively easy to manufacture because the redundant circuit patterns typically repeat at 50 micron intervals and thereby produce spatial frequency components spaced apart by a distance of about 1.0 millimeter, which is resolvable by conventional optical components. The Fourier transform and imaging areas are preferably of sufficient sizes to accommodate light from only the wafer die aligned with the optic axis. The spatial filter blocks the spatial frequencies of the error-free Fourier transform of such die, i.e., the spatial filter contains only intra-die interference pattern information.

The wafer is positioned in the front focal plane of the Fourier transform lens, and the patterned surface of the wafer is illuminated by a collimated laser beam. The Fourier transform pattern of the illuminated wafer surface is formed in the back focal plane of the Fourier transform lens. A previously fabricated spatial filter is positioned in the plane of the Fourier transform pattern and effectively stops the light transmission from the redundant circuit patterns of the illuminated dies of the wafer but allows the passage of light originating from possible defects.

The inverse Fourier transform lens receives the light either transmitted through or reflected by the spatial filter and performs the inverse Fourier transform on the filtered light diffracted by the illuminated wafer area. Whether the spatial filter is of a type that transmits or reflects light depends on the embodiment of inspection system in which it is incorporated. The filtered image strikes the surface of a two-dimensional photodetector array which detects the presence of light corresponding to defects in only the illuminated on-axis wafer die. The photodetector array is centrally positioned about the optic axis and has a light-sensitive surface area of insufficient size to cover the image plane area in which the defect image corresponding to the on-axis die appears. The inspection of all possible defects in the portions of the wafer surface having many redundant circuit patterns is accomplished by mounting the wafer onto a two-dimensional translation stage and moving the stage so that the illumination area defined by the laser beam continuously scans across the wafer surface from die to die until the desired portions of the wafer surface have been illuminated. The use of a time delay integration technique permits continuous stage movement and inspection of the portions of the wafer surface having many redundant circuit patterns in a stripe-to-stripe raster scan fashion.

The present invention is advantageous because the spatial filter need not be fabricated with the use of an error-free specimen wafer. The reason is that any defects present in such a wafer would produce light of insufficient intensity to expose the spatial filter recording medium.

The present invention detects defects in a specimen pattern by using only intra-die information corresponding to areas of the specimen pattern having many redundant circuit patterns. The premises underlying the inspection method of the present invention are that inter-die interference pattern information is unnecessary if only areas of many redundant patterns are inspected and that inspection of only such areas provides sufficient statistical sampling to determine the defect distribution for the entire specimen pattern.

Additional objects and advantages of the present invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the optical components of a first preferred embodiment of the defect inspection system of the present invention.

FIG. 2 is a diagram of a semiconductor wafer comprising a regular array of normally identical dies of the type suitable for defect inspection by the systems of FIGS. 1 and 6.

FIGS. 3A-3C are photographs of an exemplary single die of the semiconductor wafer of FIG. 2 showing within such die a highly redundant circuit pattern for consecutively increasing magnifications.

FIG. 4 is a simplified diagram showing the asymmetry of the Fourier transform and inverse Fourier transform lens system incorporated in the defect inspection system of FIG. 1.

FIG. 5 is a diagram showing the optical elements of the lens system of FIG. 4.

FIG. 6 is a schematic diagram of the optical components of a second preferred embodiment of the defect inspection system of the present invention.

FIG. 7 is a cross sectional view of the spatial filter employed in the defect inspection system of FIG. 6.

FIG. 8 shows the optical components of the Fourier transform and the inverse Fourier transform lens system incorporated in the defect inspection system of FIG. 6.

FIG. 9 is an isometric view of the scanning mechanism for detecting the presence and locations of defects in the semiconductor wafer of FIG. 2.

FIG. 10A is an enlarged fragmentary view showing three stripe regions in the lower left-hand corner of the semiconductor wafer of FIG. 9.

FIG. 10B is an enlarged, not-to-scale view of the stripe regions of FIGS. 9 and 10A that shows the raster scan path followed by the scanning mechanism of FIG. 9 relative to a light sensitive detector to detect defect images in a defect image field.

FIG. 11 is a diagram showing an array of pixel elements in the defect image field under tenfold magnification and an array of light detecting elements of a charge-coupled device used in the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a schematic diagram of a first preferred embodiment of an inspection system 10 of the present invention that is designed to detect semiconductor wafer defects having a diameter of about one-quarter micron or larger in the presence of a periodic structure comprising many redundant circuit patterns. FIG. 2 is a diagram of a semiconductor wafer 12 of the type inspection system 10 is designed to inspect for defects. Wafer 12 includes a regular array of normally identical dies 14 of which each has at least about twenty redundant circuit patterns 16 along each of the X-axis 18 and Y-axis 20. Each die 14 is typically of square shape with about 3 millimeter sides. FIGS. 3A-3C are photographs of an exemplary single die 14 showing highly repetitive circuit pattern within such element for consecutively increasing magnifications. Although they are of rectangular shape as shown in FIGS. 3A-3C, circuit patterns 16 are assumed for purposes of simplifying the following discussion to be of square shape with about 50 micron sides.

With reference to FIG. 1, inspection system 10 includes a laser source 22 that provides a nearly collimated beam of 442.5 nanometer monochromatic light rays 24 that strike a lens 26 that converges the light rays to a point 28 located in the back focal plane of lens 26. The light rays 30 diverging from focal point 28 strike a small mirror 32 that is positioned a short distance from focal point 28 to reflect a relatively narrow circular beam of light toward a Fourier transform lens section 34, which is shown in FIG. 1 as a single element but which is implemented in five lens elements as will be further described below. Mirror 32 obscures a small region in the center of the Fourier transform plane defined by lens section 34. The size of the obscured region is sufficiently small so that defect information, which is located everywhere in the Fourier transform plane, is only insignificantly blocked by mirror 32.

The effective center of Fourier transform lens section 34 is positioned a distance of slightly less than one focal length away from mirror 32 to provide collimated light rays 36 that strike the patterned surface of wafer 12. Wafer 12 is mounted in a chuck 38 that constitutes part of a two-dimensional translation stage 40. Wafer 12 is positioned in the object or front focal plane 42 of lens section 34, and the collimated light rays 36 illuminate the patterned surface of wafer 12. The collimated light rays 36 illuminate a 20 millimeter diameter area of the surface of wafer 12. The light rays 44 diffracted by the illuminated area of wafer 12 pass through lens section 34 and form the Fourier transform pattern of the illuminated wafer surface in the back focal plane 46 of lens section 34.

The Fourier transform pattern comprises an array of bright spots of light that are distributed in back focal plane 46 in a predictable manner. The 20 millimeter diameter illuminated area of wafer 12 provides a Fourier transform pattern of sufficient accuracy because it is formed from many redundant circuit patterns. The design of lens section 34 is, however, such that it has only a 3 millimeter object field diameter to form in the image plane 60 an essentially aberration-free image of defects in the semiconductor wafer. An entire die can be inspected for defects because translation stage 40 moves the die through the illuminated area. Therefore, a relatively large area of wafer 12 is illuminated to develop an accurate Fourier transform pattern of the redundant circuit patterns, but 54 264 is shown in phantom for inspection area 250 in this position.) X-stage 258 and Y-stage 256 retrace wafer 12 along path segment 284a to position start location 286 at optical window 264. The potential wells of light detecting elements 260 are cleared during this time in preparation for the scan of the next adjacent stripe region 254. The scan and retrace of the second and succeeding stripe regions 254 proceed as described above.

It will be obvious to those having skill in the art that many changes may be made in the above-described details of the preferred embodiment of the present invention without departing from the underlying principles thereof. For example, a photomask, instead of a semiconductor wafer, can be inspected for defects. Ispection Inspection systems 10 and 100 would, however, have to be modified to direct the laser light for transmission through the photomask. As a second example, a polarizing beam splitter of the cube type can be substituted for the plate-type beam splitter 116 employed in inspection system 100. A cube type beam splitter would reduce background noise resulting from light reflection but would require a change in the prescription of lens system 200 to reduce spherical aberrations introduced by such a beam splitter. The scope of the present invention should be determined, therefore, only by the following claims.

Claims

1. In an imaging system that includes first and second lenses positioned along an optic axis, the first lens producing from a specimen a spatial frequency spectrum whose frequency components can be selectively filtered and the second lens producing an image of defects present in the specimen, a method of detecting defects in a specimen that includes an array of normally substantially identical dies, each of the dies having many redundant circuit patterns, comprising:

illuminating plural die circuit patterns;

generating a light pattern representing substantially the Fourier transform pattern of the illuminated die circuit patterns, the light pattern including intra-die interference pattern information;

positioning an optical filter to receive the light pattern and to block spatial frequency components thereof, the optical filter having relatively transparent and relatively nontransparent portions, the relatively nontransparent portion conforming to the Fourier transform pattern of an error-free reference pattern corresponding to the die circuit patterns;

collecting spatial frequency components not blocked by the optical filter to form an image of the defects, the collected spatial frequency components corresponding to a small number of die circuit patterns relative to the number of die circuit patterns in the array of dies and residing in a spatial region intercepting the optic axis; and

processing unblocked intra-die spatial frequency components to determine the location and size of a possible defect in the a die.

2. The method of claim 1 which further comprises changing the position of the specimen relative to the position of the optic axis so that different ones of the die circuit patterns are positioned within the spatial region intercepted by the optic axis, thereby to process the intra-die spatial frequency components of the different ones of the die circuit patterns.

3. The method of claim 1 in which the processing of the unblocked intra-die spatial frequency components is accomplished by positioning a light sensitive detector surface generally centrally about the optic axis, the light sensitive detector surface having an area that is smaller than the surface area of the image of the defects.

4. The method of claim 1 in which the first and second lenses cooperate to receive light diffracted by, and provide an image from the spatial frequency components corresponding to, the illuminated die circuit patterns.

5. The method of claim 4 in which the first lens comprises a first lens section of plural elements and the second lens comprises a second lens section of plural elements, the first and second lens sections forming a near diffraction-limited lens system of asymmetric character.

6. The method of claim 1 in which the illuminating means emits nearly collimated light, the method further comprising:

defining with respect to the specimen plural adjacent stripes, each stripe including a series of adjacent dies;

moving the specimen and the collimated light relative to each other along the length of each stripe to illuminate the die circuit patterns in proximal position to the optic axis; and

processing the unblocked intra-die spatial frequency components corresponding to the die circuit patterns in proximal position to the optic axis.

7. The method of claim 6 in which the specimen is movable and the collimated light remains fixed relative to the optic axis.

8. The method of claim 1 in which the relatively transparent and relatively nontransparent portions of the optical filter are developed by computer generation techniques.

9. The method of claim 1 in which the relatively transparent and relatively nontransparent portions of the optical filter are developed by positioning a recording medium in the location of the Fourier transform pattern and exposing the recording medium to light propagating from the specimen.

10. The method of claim 1 in which the collected spatial frequency components correspond to fewer than all of the illuminated die circuit patterns.

11. In an imaging system that includes first and second lenses positioned along an optic axis, the first lens producing from a specimen a spatial frequency spectrum whose frequency components can be selectively filtered and the second lens producing an image of defects present in the specimen, a method of detecting defects in a specimen that includes an array of normally substantially identical dies occupying a first area of the specimen, each of the dies having many redundant circuit patterns, comprising:

illuminating a second area of the specimen, the second area containing die circuit patterns and intercepting the optic axis;

generating a light pattern representing substantially the Fourier transform pattern of the illuminated die circuit patterns, the light pattern including intra-die interference pattern information;

positioning an optical filter to receive the light pattern and to block spatial frequency components thereof, the optical filter having relatively transparent and relatively nontransparent portions, the relatively nontransparent portion conforming to the Fourier transform pattern of an error-free reference pattern corresponding to the die circuit patterns;

collecting spatial frequency components not blocked by the optical filter to form an image of the defects; and

processing only unblocked intra-die spatial frequency components to determine the location and size of a possible defect in the a die.

12. The system of claim 11 in which the size of the first area differs from that of the second area.

13. The system of claim 12 in which the second area is substantially smaller than the first area.

14. The system of claim 12 in which the second area contains more than one die.

15. In an imaging system that includes first and second lenses positioned along an optic axis, the first lens producing from a specimen a spatial frequency spectrum whose frequency components can be selectively filtered and the second lens producing an image of defects present in the specimen, a method of detecting defects in a specimen that includes an array of normally substantially identical dies occupying a first area of the specimen, each of the dies having many redundant circuit patterns, comprising:

illuminating die circuit patterns included within a second area of the specimen;

generating a light pattern representing substantially the Fourier transform pattern of the illuminated die circuit patterns, the light pattern including intra-die interference pattern information;

positioning an optical filter to receive the light pattern and to block spatial frequency components thereof, the optical filter having relatively transparent and relatively nontransparent portions, the relatively nontransparent portion conforming to the Fourier transform pattern of an error-free reference pattern corresponding to the die circuit patterns;

collecting spatial frequency components not blocked by the optical filter to form an image of the defects, the collected spatial frequency components corresponding to fewer than all of the illuminated die circuit patterns; and

processing the unblocked intra-die spatial frequency components to determine the location and size of a possible defect in the die.

16. The method of claim 15 in which the second area is substantially smaller than the first area.

17. In an imaging system that includes first and second lenses positioned along an optic axis, the first lens producing from a specimen a spatial frequency spectrum whose frequency components can be selectively filtered and the second lens producing an image of defects present in the specimen, a method of detecting defects in a specimen that includes an array of normally substantially identical dies, each of the dies having many redundant circuit patterns, comprising:

illuminating plural die circuit patterns;

generating a light pattern representing substantially the Fourier transform pattern of the illuminated die circuit patterns, the light pattern including intra-die interference pattern information;

positioning an optical filter to receive the light pattern and to block spatial frequency components thereof, the optical filter having relatively transparent and relatively nontransparent portions, the relatively nontransparent portion conforming to the Fourier transform pattern of an error-free reference pattern corresponding to the die circuit patterns;

collecting spatial frequency components not blocked by the optical filter within a region proximal to the optic axis to form an image of the defects; and

processing unblocked intra-die spatial frequency components to determine the location and size of a possible defect in the a die, the processed spatial frequency components corresponding to a small number of die circuit patterns relative to the number of die circuit patterns in the array of dies and lying in a spatial region intercepting the optic axis.

18. An optical system for detecting defects in a specimen pattern of a type that includes an array of normally essentially identical dies of which each has many redundant circuit patterns and which occupy a first area of the specimen, the system comprising:

illuminating means for illuminating a second area of the specimen, the second area being occupied by plural die circuit patterns;

pattern generating means for generating a light pattern representing substantially the Fourier transform pattern of the illuminated die circuit patterns, the light pattern including intra-die interference pattern information;

optical filter means receiving the light pattern for blocking spatial frequency components thereof, the optical filter means having relatively transparent and relatively nontransparent portions, the relatively nontransparent portion conforming to the Fourier transform of an error-free reference pattern corresponding to the die circuit patterns;

collecting means for collecting the spatial frequency components not blocked by the optical filter means; and

processing means for processing only the unblocked intra-die spatial frequency components to determine the location and size of a possible defect in the die.

19. The system of claim 18 in which the illuminating means emits nearly collimated light and which further comprises positioning means for changing the position of the specimen relative to the position of the collimated light so that different ones of the die circuit patterns occupy the second area of the specimen illuminated by the collimated light, thereby to process the intra-die spatial frequency components of the different ones of the die circuit patterns.

20. The system of claim 18 in which the pattern generating means and the collecting means comprises respective first and second lenses positioned along an optic axis that intersects the second area of the specimen illuminated by the illuminating means.

21. The system of claim 18 in which the pattern generating means and the collecting means comprise respective first and second lenses that cooperate to receive light diffracted by, and provide an image from the spatial frequency components corresponding to, the illuminated die circuit patterns.

22. The system of claim 21 in which the first lens comprise a first lens section of plural elements and the second lens comprises a second lens section of plural elements, the first and second lens sections forming a near diffraction-limited lens system of asymmetric character.

23. The system of claim 21 in which the first lens comprises a first lens section of plural elements and the second lens comprises a second lens section of plural elements, the first lens section forming the Fourier transform pattern and cooperating with the second lens section to provide a magnified image of the defects in the illuminated die circuit patterns.

24. The system of claim 18 in which the pattern generating means and the collecting means comprise a folded Fourier transform optical system that receives light diffracted by, and provides an image from the spatial frequency components corresponding to, the illuminated die circuit patterns.

25. The system of claim 24 in which the specimen comprises a semiconductor wafer.

26. The system of claim 24 in which the optical filter means comprises a liquid crystal layer.

27. The system of claim 26 in which the relatively nontransparent portion of the liquid crystal layer scatters light of the spatial frequencies incident to it.

28. The system of claim 18 in which the optical filter means comprises a liquid crystal layer.

29. The system of claim 28 in which the relatively nontransparent portion of the liquid crystal layer scatters light of the spatial frequencies incident to it.

30. The system of claim 18 in which the optical filter means comprises exposed light sensitive material.

31. The system of claim 18 in which the optical filter means comprises a lens assembly that has an aperture of at least ±15°.

32. The system of claim 18 in which the Fourier transform light pattern represents the Fourier transform image.

33. The system of claim 18 in which the specimen comprises a semiconductor wafer.

34. The system of claim 18 in which the second area is substantially smaller than the first area.

35. The system of claim 18 in which the illuminating means emits nearly collimated light and the processing means comprises a light sensitive detector having a light sensitive surface positioned generally centrally about the optic axis, : the light detector including plural light detecting elements arranged in a first array of rows and columns and defining in the light pattern plural adjacent stripe regions each of which includes plural pixel elements arranged in a second array of rows and columns, and each light detecting element being operable to provide a measured energy value corresponding to the amount of light present in any one of the pixel elements, and the system further comprising:

positioning means for positioning the specimen relative to the collimated light to scan the light detecting means along a stripe region of the light pattern so that in succession each light detecting element in one column of the first array traverses and acquires an energy value corresponding to the amount of light present in a pixel element in one column of the second array;

accumulating means to accumulate a total energy value proportional to the sum of the energy values acquired for the pixel element by all of the light detecting elements in the one column of the first array; and

means to determine from the total energy value whether the amount of light in the pixel element represents a defect in the specimen subject.

36. The system of claim 35 in which the light detector comprises a charge-coupled device.

37. The system of claim 35 in which the collimated light remains stationary and the positioning means scans each one of the stripe regions across the light sensitive surface in a serial manner.

38. The system of claim 37 in which the positioning means continuously moves each stripe region across the collimated light.

39. The system of claim 35 in which the first array has a first row and N total number of rows and which further comprises position-detecting means for detecting the position of the first array relative to the stripe region, the position-detecting means cooperating with the accumulating means so that each one of the light detecting elements in the first row of the one column never accumulates more than one energy value for any one of the pixel elements of the second array with which it becomes aligned, and each one of the light detecting elements in the Nth row of the one column has accumulated N number of energy values for any one of the pixel elements with which it becomes aligned.

40. In an imaging system that includes first and second lenses positioned along an optic axis, the first lens producing from a specimen a spatial frequency spectrum whose frequency components can be selectively filtered and the second lens producing an image of defects present in the specimen, a method of detecting nonperiodic defects in a specimen that includes one or more dies having many redundant circuit patterns, comprising:

illuminating plural die circuit patterns;

generating a light pattern representing substantially the Fourier transform pattern of the illuminated die circuit patterns, the light pattern including intra-die interference pattern information;

positioning an optical filter to receive the light pattern and to block spatial frequency components thereof, the optical filter having relatively transparent and relatively nontransparent portions, the relatively nontransparent portion conforming to the Fourier transform pattern of an error-free reference pattern corresponding to the die circuit patterns;

collecting spatial frequency components not blocked by the optical filter to form an image of the defects, the collected spatial frequency components corresponding to a small number of die circuit patterns relative to the number of illuminated die circuit patterns and residing in a spatial region intercepting the optic axis; and

processing unblocked intra-die circuit pattern spatial frequency components to determine the presence of a possible nonperiodic defect in a die.

41. The method of claim 40 which further comprises changing the position of the specimen relative to the position of the optic axis so that different ones of the die circuit patterns are positioned within the spatial region intercepted by the optic axis, thereby to process the intra-die spatial frequency components of the different ones of the die circuit patterns. 42. The method of claim 40 in which the processing of the unblocked intra-die spatial frequency components is accomplished by positioning a light sensitive detector surface generally centrally about the optic axis, the light sensitive detector surface having an area that is smaller than the surface area of the image of the defects. 43. The method of claim 40 in which the first and second lenses cooperate to receive light diffracted by, and provide an image from the spatial frequency components corresponding to, the illuminated die circuit patterns. 44. The method of claim 40 in which the illuminating means emits substantially collimated light, the method further comprising:

defining with respect to the specimen adjacent stripes for scanning through many redundant die circuit patterns;

moving the specimen and the collimated light relative to each other along the length of each stripe to illuminate die circuit patterns in proximal position to the optic axis; and

processing the unblocked intra-die spatial frequency components corresponding to the die circuit patterns in proximal position to the optic axis. 45. The method of claim 44 in which the stripe for scanning includes more than one area of redundant die circuit patterns. 46. The method of claim 44 in which the stripe

for scanning traverses through one or more dies. 47. An optical system for detecting nonperiodic defects in a specimen pattern of a type that includes one or more dies having many redundant circuit patterns occupying a first area of the specimen, the system comprising:

illuminating means for illuminating a second area of the specimen, the second area being occupied by plural redundant die circuit patterns;

pattern generating means for generating a light pattern representing substantially the Fourier transform pattern of the illuminated die circuit patterns, the light pattern including intra-die interference pattern information;

optical filter means receiving the light pattern for blocking spatial frequency components thereof, the optical filter means having relatively transparent and relatively nontransparent portions, the relatively nontransparent portion conforming to the Fourier transform of an error-free reference pattern corresponding to the die circuit patterns;

collecting means for collecting the spatial frequency components not blocked by the optical filter means; and

processing means for processing the unblocked intra-die spatial frequency components to determine the presence of a possible nonperiodic defect in a

die. 48. The system of claim 47 in which the illuminating means emits substantially collimated light and in which the collecting means collects spatial frequency components residing in a spatial region intercepting the optic axis, the spatial frequency components corresponding to a smaller number of die circuit patterns relative to the number of illuminated die circuit patterns and being in proximal position to the optic axis. 49. The system of claim 47 in which further comprises positioning means for changing the position of the specimen so that different ones of the die circuit patterns are positioned within the second area, thereby to process the intra-die spatial frequency components of the different ones of the die circuit patterns. 50. The system of claim 47 in which the collecting means collects spatial frequency components corresponding to fewer than all of the illuminated die circuit patterns. 51. The system of claim 47 in which the illuminating means emits substantially collimated light and in which the pattern generating means and the collecting means comprise respective first and second lenses that cooperate to receive light diffracted by, and provide an image from the spatial frequency components residing in a spatial region intercepting the optic axis and corresponding to, a smaller number of die circuit patterns relative to the number of illuminated die circuit patterns and positioned proximally to the optic axis, the system further comprising:

positioning means for changing the position of the specimen relative to the position of the collimated light to scan the specimen in stripes so that different ones of the die circuit patterns serially occupy the second area. 52. The system of claim 51 in which the positioning means scans a stripe that traverses one or more dies including one or more areas having many redundant die circuit patterns. 53. The system of claim 51 in which the first lens comprises a first lens section of plural elements and the second lens comprises a second lens section of plural elements, the first and second lens sections forming a near diffraction-limited lens system of asymmetric character.

. The system of claim 51 in which the first lens comprises a first lens section of plural elements and the second lens comprises a second lens section of plural elements, the first lens section forming the Fourier transform pattern and cooperating with the second lens section to provide a magnified image of the defects in the illuminated die circuit

patterns. 55. In an imaging system that includes first and second lenses positioned along an optic axis, the first lens producing from a specimen a spatial frequency spectrum whose frequency components can be selectively filtered and the second lens producing an image of defects present in the specimen, a method of detecting nonperiodic defects in a specimen that includes one or more dies having many redundant die circuit patterns, comprising:

illuminating a small number of die circuit patterns relative to the number of die circuit patterns in the specimen;

generating a light pattern representing substantially the Fourier transform pattern of the illuminated die circuit patterns, the light pattern including intra-die interference pattern information;

positioning an optical filter to receive the light pattern and to block spatial frequency components thereof, the optical filter having relatively transparent and relatively nontransparent portions, the relatively nontransparent portion conforming to the Fourier transform pattern of an error-free reference pattern corresponding to the die circuit patterns;

collecting spatial frequency components not blocked by the optical filter to form an image of the defects, the collected spatial frequency components residing in a spatial region intercepting the optic axis and corresponding to a small number of die circuit patterns relative to the number of illuminated die circuit patterns and being in proximal position to the optic axis; and

processing the unblocked intra-die spatial frequency components to determine the presence of a possible nonperiodic defect.

. The method of claim 55 which further comprises changing the position of the specimen relative to the position of the optic axis so that spatial frequency components of different ones of the die circuit patterns are collected within the spatial region intercepted by the optic axis, thereby to process the intra-die spatial frequency components of the different ones of the die circuit patterns. 57. The method of claim 55 in which the illuminating means emits nearly collimated light, the method further comprising:

defining with respect to the specimen adjacent stripes for scanning through many redundant die circuit patterns;

moving the specimen and the collimated light relative to each other along the length of each stripe to illuminate the circuit patterns; and

processing the unblocked intra-die frequency components corresponding to the circuit patterns in proximal position to the optic axis. 58. The method of claim 57 in which the stripe for scanning traverses through one or more dies.