Patent Publication Number: US-2011075134-A1

Title: Defect Inspection Method and System

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 12/366,956, filed Feb. 6, 2009, which is a continuation of U.S. application Ser. No. 11/626,925, filed Jan. 25, 2007, now U.S. Pat. No. 7,492,452, issued on Feb. 17, 2009, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and system for inspecting the occurrence of a foreign matter or defect in a device manufacturing process, wherein a foreign matter existent on a thin-film substrate, a semiconductor substrate, or a photomask, or a defect occurring in circuit patterns is detected in the course of manufacturing a semiconductor chip or a liquid crystal product, and the detected foreign matter or defect is analyzed in order to take measures. 
     2. Description of the Related Art 
     In a semiconductor manufacturing process, the presence of a foreign matter on a semiconductor substrate (wafer) causes a defective such as imperfect insulation of wiring or a short circuit thereof. Furthermore, due to the trend of semiconductor devices to a more and more microscopic structure, a more microscopic foreign matter brings about imperfect insulation of a capacitor or destruction of a gate oxide film. The foreign matters are mixed in various states because of various causes. Namely, the foreign matters may be produced by a movable unit included in transportation equipment or a human body, produced in processing equipment due to reaction of a process gas or the like, or originally mixed in a chemical agent or material. 
     Likewise, in a process of manufacturing a liquid crystal display device, if a defective pattern is produced in a work due to a foreign matter, the work cannot be adopted as a display device. The same applies to a manufacturing process of a printed circuit board. The mixture of a foreign matter causes a short circuit or a defective connection in patterns. Under such a background, in the case of semiconductor manufacturing, a plurality of foreign matter inspection systems may be disposed relative to each production line. Thus, the presence of a foreign matter is discovered in the earliest possible stage and fed back to the manufacturing process, whereby a yield in manufacturing of a semiconductor device is improved. 
     As one of techniques for detecting a foreign matter on a semiconductor substrate, Japanese Unexamined Patent Publication No. 62-89336 (Prior Art 1) has disclosed a method capable of highly sensitively and highly reliably inspecting the presence of a foreign matter and the occurrence of a defect by discarding false or misleading information acquired from patterns. According to the method, light scattered from a foreign matter, which adheres to a semiconductor substrate, after laser light is irradiated to the semiconductor substrate is detected, and then compared with a result of inspection conducted on a semiconductor substrate of the same type that has been inspected immediately previously. Moreover, as disclosed in Japanese Unexamined Patent Publication No. 63-135848 (Prior Art 2), a method is known in which light scattered from a foreign matter, which adheres to a semiconductor substrate, after laser light is irradiated to the semiconductor substrate is detected, and the detected foreign matter is analyzed using an analyzing technique such as laser photoluminescence or secondary X-ray analysis (XMR). 
     Moreover, disclosed as a technique for inspecting the presence of a foreign matter is a method for irradiating coherent light to a wafer, removing light, which is emitted from repetitive patterns on the wafer, using a spatial filter, and intensifying light reflected from a foreign matter or defect, which lacks repetitiveness, so as to detect the foreign matter or defect. Moreover, a foreign matter inspection system that irradiates light to a major group of straight lines, which is included in circuit patterns formed on a wafer, at an angle of 45°, for fear light having undergone zero-order diffraction due to the major group of straight lines may enter an aperture for an objective lens is described in Japanese Unexamined Patent Publication No. 1-117024 (Prior Art 3). In the Prior Art 3, a spatial filter is used to intercept light reflected from a group of straight lines other than the major group of straight lines. 
     As prior arts concerning a defect inspection system and method for inspecting a foreign matter or the like, methods described in Japanese Unexamined Patent Publications Nos. 1-250847 (Prior Art 4) and 2000-105203 (Prior Art 5) are known. In particular, as for the Prior Art 5, the patent publication describes that detective optical systems are switched in order to change detectable pixel sizes. As for technologies for measuring the size of a foreign matter, a method is disclosed in Japanese Unexamined Patent Publication No. 2001-60607 (Prior Art 6). 
     SUMMARY OF THE INVENTION 
     However, the foregoing Prior Arts 1 to 5 cannot readily, highly sensitively, and quickly detect a microscopic foreign matter or defect on a substance on which repetitive patterns or non-repetitive patterns coexist. Namely, the Prior Arts 1 to 5 are disadvantageous in a point that detective sensitivity (a minimum dimension of a foreign matter to be detected) is low except that for the repetitive patterns such as those realizing memory cells or the like. Moreover, the Prior Arts 1 to 5 are disadvantageous in a point that the detective sensitivity is low relative to a microscopic foreign matter or defect of about 0.1 μm in a diameter existing in a highly densely patterned area. Moreover, the Prior Arts 1 to 5 are disadvantageous in a point that the detective sensitivity is low relative to a foreign matter or defect which causes a short circuit of wiring, or a foreign matter shaped like a thin film. Moreover, the Prior Art 6 is disadvantageous in a point that the precision in measurement of a foreign matter or defect is low. Moreover, the Prior Art 6 is disadvantageous in a point that the detective sensitivity is low relative to a foreign matter on the surface of a wafer coated with a transparent thin film. 
     The present invention addresses the foregoing problems. An object of the present invention is to provide a defect inspection method and system capable of inspecting an object of inspection on which repetitive patterns and non-repetitive patterns coexist so as to detect a microscopic foreign matter or defect quickly and highly precisely. In particular, the present invention provides a defect inspection method and system capable of stably detecting a microscopic foreign matter or defect while minimizing a difference in sensitivity between inspective areas, that is, a memory unit that includes many repetitive patterns and a logic unit in which non-repetitive patterns exist. 
     Moreover, another object of the present invention is to provide a defect inspection method and system that are attempted to improve the sensitivity in detecting a defect in a memory unit including many repetitive patterns. Herein, when a setting is designated for intercepting light, which is diffracted by patterns formed on a substrate to be inspected, at a position of a Fourier transform in a detective optical system using a spatial filter, the setting may be designated by employing either light diffracted by actual patterns or design data representing patterns. The setting of the spatial filter is designated prior to inspection of each of a plurality of memory units included in the same die. When each of the memory units is inspected, the setting of the spatial filter is automatically designated in order to highly sensitively detect a microscopic foreign matter or defect in the memory unit. 
     To be more specific, in one aspect of the present invention, there is provided a defect inspection system including: an illuminative optical system capable of irradiating illuminating luminous fluxes, which are emitted from an illuminative light source and have different wavelengths, to the surface of a substrate to be inspected in mutually different directions and at mutually different tilt angles; an objective lens on which light reflected or scattered from a defect on the substrate to be inspected is converged; a detective optical system realized with an image-formation optical system that separates the wavelengths of the reflected or scattered light converged on the objective lens from one another, and causes the different wavelength components to form images on the light receiving surfaces of respective photodetectors; and a signal processing system that converts image signals produced by the respective photodetectors included in the detective optical system into digital image signals, and detects the defect on the basis of the digital image signals produced by the converter. 
     In another aspect of the present invention, the detective optical system included in the defect inspection system includes a mechanical unit that varies an image-formation power by keeping constant the relative distances between a Fourier-transform image, which is formed between the objective lens and the image formation optical system, and the substrate to be inspected and photodetectors. 
     In another aspect of the present invention, the detective optical system included in the defect inspection system includes a spatial filter that is interposed between the objective lens and image formation optical system and that has the ability to intercept light of a specific wavelength out of the light reflected or scattered from the substrate to be inspected. Moreover, according to the present invention, the defect inspection system includes an arrangement that designates the conditions for the spatial filter, which is included in the detective optical system and interposed between the objective lens and image formation optical system, on the basis of design data representing patterns. 
     In another aspect of the present invention, there is provided a defect inspection system including: an illuminative optical system capable of irradiating an illuminating luminous flux, which is emitted from an illumination light source, to the surface of a substrate to be inspected at a high tilt angle and a low tilt angle that can be switched; a detective optical system including an objective lens which is disposed in a direction optimal for detecting a foreign matter or defect on the object of inspection and on which light reflected or scattered from the foreign matter or defect is converged, an image formation optical system that causes the reflected or scattered light, which is converted on the objective lens, to form an image, and a photodetector that converts the reflected or scattered light, which is caused to form an image by the image formation optical system, into a signal; an ND converter that when the illuminative optical system illuminates the substrate to be inspected at the high and low tilt angles, converts an image signal produced by the photodetector included in the detective optical system into a digital image signal; a defect detection unit that detects the defect on the basis of the digital image signal produced by the ND converter; and a means for verifying the detected foreign matter. 
     In another aspect of the present invention, there is employed a pulsed laser, which emits light that falls within the ultraviolet region, as a low-angle illumination light source. Moreover, one pulse of laser light emitted from the pulsed laser light source is split into a plurality of pulses in order to decrease the peak value of the laser light. The resultant laser light is irradiated to a specimen, whereby the damage the specimen incurs is reduced. 
     The aforesaid and other objects of the present invention and the features and advantages thereof will be apparent from the following more particular description of preferred embodiments of the invention illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram schematically showing the configuration of a defect inspection system in accordance with the first embodiment; 
         FIG. 2A  is a side view for use in explaining the disposition of an illuminative optical system shown in  FIG. 1 ; 
         FIG. 2B  is a perspective view schematically showing the elements of a low-angle illumination optical system included in the first embodiment; 
         FIG. 3A  is a perspective view showing a conically curved lens employed in an illuminative optical system that obliquely illuminates a linear area  201 - 1  on a wafer; 
         FIG. 3B  is a perspective view showing a cylindrical lens employed in an illuminative optical system that longitudinally illuminates the linear area  201 - 1  on the wafer; 
         FIG. 3C  is a perspective view showing a cylindrical lens employed in an illuminative optical system that laterally illuminates the linear area  201 - 1  on the wafer; 
         FIG. 4A  is a plan view of a high-angle illumination optical system; 
         FIG. 4B  is a front view of the high-angle illumination optical system; 
         FIG. 4C  is an A-A sectional view of the high-angle illumination optical system; 
         FIG. 5A  is a schematic front view of a variant of the high-angle illumination optical system; 
         FIG. 5B  is a schematic front view of another variant of the high-angle illumination optical system; 
         FIG. 6A  is a schematic front view of still another variant of the high-angle illumination optical system; 
         FIG. 6B  is a schematic front view of the variant shown in  FIG. 6B ; 
         FIG. 7  is a front view schematically showing the elements of an objective lens; 
         FIG. 8A  shows a diffraction pattern produced by light reflected or scattered from a wafer when a pupil of an objective lens is viewed through a pupil viewing optical system; 
         FIG. 8B  shows a state in which the diffraction pattern observed through the pupil viewing optical system is intercepted by a one-dimensional spatial filter; 
         FIG. 8C  shows a diffraction pattern observed when the pupil of the objective lens is viewed through the pupil viewing optical system and the wafer is illuminated in multiple directions; 
         FIG. 8D  shows a state in which a two-dimensional spatial filter is disposed in order to intercept the diffraction pattern shown in  FIG. 8C ; 
         FIG. 9A  is a plan view schematically showing the structure of a variable one-dimensional spatial filter; 
         FIG. 9B  is a front view schematically showing the structure of the variable one-dimensional spatial filter; 
         FIG. 9C  is a front view schematically showing the structure of a variable two-dimensional spatial filter; 
         FIG. 9D  is a plan view showing a spring structure included in the variable two-dimensional spatial filter; 
         FIG. 10  is a plan view schematically showing the configuration of a chip of a semiconductor device formed on a wafer; 
         FIG. 11A  is a front view for use in explaining in detail a detective optical system; 
         FIG. 11B  shows a waveform of a detective signal produced when light S 0  reflected or scattered from a wafer is detected; 
         FIG. 11C  shows a waveform of a detective signal produced when light reflected or scattered under illumination with laser light is selected and detected; 
         FIG. 12  is a flowchart describing an inspection procedure; 
         FIG. 13  is a block diagram showing in detail a signal processing system included in the first embodiment; 
         FIG. 14  is a block diagram showing in detail the configuration of a threshold calculation block included in the first embodiment; 
         FIG. 15  is a perspective view showing the overall configuration of a defect inspection system in accordance with the second embodiment; 
         FIG. 16  schematically shows a defect inspection system in accordance with the third embodiment including an observational optical system; 
         FIG. 17  is a plan view showing the arrangement of elements of a path branching optical system included in the fourth embodiment; 
         FIG. 18A  is a block diagram schematically showing the path branching optical system included in the fourth embodiment; 
         FIG. 18B  shows a waveform of a signal of pulsed laser light emitted from a laser light source; 
         FIG. 18C  shows a waveform of pulses demonstrating that one pulse of laser light emitted from the laser light source is split into two pulses; 
         FIG. 19A  is a block diagram schematically showing the elements of a path branching optical system included in a variant of the fourth embodiment; and 
         FIG. 19B  shows a waveform of pulses demonstrating split of a pulse. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, embodiments of the present invention will be described below. 
     A defect inspection system in accordance with the present invention highly sensitively and quickly inspects the presence of various defects including a foreign matter, a defective pattern, and a micro-scratch on a substrate to be inspected such as a wafer that is made of various materials through various manufacturing steps, and especially stably detects a defect on the surface of a thin film coated over the surface of the wafer independently of a defect in the thin film. 
     Specifically, the defect inspection system in accordance with the present invention is designed so that an angle α of irradiation and a direction φ of irradiation at or in which a slit-shaped beam  201  that is, as shown in  FIG. 2 , produced for illumination by an illuminative optical system  10  can be varied depending on an object of inspection. As shown in  FIG. 1 , the elements of a detective optical system  20  are arranged so that the surface of the object of inspection and the light receiving surface of a detector  26  will have a relationship of image formation. Moreover, a power for image formation offered by the detective optical system  20  is made variable so that the size of a defect-detected pixel can be determined according to the size of a defect to be detected. Thus, inspection is carried out. 
     Furthermore, the defect inspection system in accordance with the present invention includes a facility that distinguishes a kind of detected defect by recognizing a difference in light, which is scattered from a defect by irradiating illumination light waves at different angles of irradiation, as a characteristic quantity. 
     The embodiments of the defect inspection system in accordance with the present invention will be described concretely. The embodiments will be described on the assumption that a defect such as a small or large foreign matter or a micro-scratch on a semiconductor wafer or a transparent film coated over the wafer, a foreign matter in the transparent film, or a defective pattern is inspected. However, the present invention is not limited to the semiconductor wafer but can be applied to a thin-film substrate, a photomask, a thin-film transistor (TFT), or a plasma display panel (PDP). 
     First Embodiment 
       FIG. 1  shows the configuration of a defect inspection system in accordance with the first embodiment. The defect inspection system includes mainly an illuminative optical system  10 , a variable-power detective optical system  20 , a transportation system  30 , a signal processing system  40 , and an overall control unit  50  that controls the entire defect inspection system. 
     The transportation system  30  includes an X stage  31 - 1 , a Y stage  21 - 2 , a Z stage  32 , and a θ stage  33  that are used to move a placement table bearing a substrate  1  to be inspected such as a wafer that is made of various materials through various manufacturing steps, and a drive circuit  35  that controls the stages. 
     The illuminative optical system  10  includes an illumination light source  12  whose light falls in a wide wavelength band, a laser light source  11 , a beam enlargement optical system  16 , mirrors  254  and  256 , and a lens  255 . Light emitted from the laser light source  11  is enlarged to a certain size by the beam enlargement optical system  16 , and then irradiated to the substrate  1  to be inspected in a plurality of oblique directions via the mirror  254 , lens  255 , and mirror  256 . 
     The detective optical system  20  includes an objective lens  21 , a spatial filter  22 , an image formation lens  23 , an optical filter  25 , a beam splitter  29 , and photodetectors  26   a  and  26   b  such as time-delay integration (TDI) image sensors. 
     Processing circuits  40   a  and  40   b  included in the signal processing system  40  manipulate image signals produced by the photodetectors  26   a  and  26   b  respectively so as to detect a defect or foreign matter. The beam splitter  29  reflects light that falls in a specific wavelength band and transmits light having the other wavelengths. 
     An observational optical system  60  includes a lens  61 , a polarization beam splitter  62 , an illumination light source  63 , and an imaging means  64 . Herein, p-polarized light emitted from the illumination light source  63  is reflected from the polarization beam splitter  62 , has thus the path thereof angled toward the wafer  1 , and is then converged on the lens  61 . Consequently, the surface of the wafer  1  is illuminated. Out of light reflected or scattered from the wafer  1 , light incident on the lens  61  falls on the polarization beam splitter  62 . The s-polarized light component of the light passes through the polarization beam splitter  62 , and forms an image which is picked up by the imaging means  64 . The wafer  1  is inspected in advance by other inspection system, and the presence or absence of a detected foreign matter and the shape thereof are verified through the observational optical system. Moreover, the observational optical system is used to observe a foreign matter or defect detected by the variable-power detective optical system  20 . 
     The overall control unit  50  designates conditions for inspection or the like, and controls the illuminative optical system  10 , variable-power detective optical system  20 , transportation system  30 , and signal processing system  40 . The overall control unit  50  includes an input/output means  51  (including a keyboard and a network), a display means  52 , and a memory unit  53 . Reference numeral  55  denotes a storage means (server) in which design data including data that represents circuit patterns formed on the surface of the substrate  1  to be inspected is stored. The design data is used to form a spatial optical image. 
     The defect inspection system includes an automatic focusing control system (not shown) so that an image of the surface of the wafer  1  will be formed on the light receiving surfaces of the respective photodetectors  26   a  and  26   b.    
     The present inspection system is designed to be able to illuminate the surface of the substrate  1  to be inspected in a plurality of directions. As shown in  FIG. 2A , the illuminative optical system  10  includes, as described in the Japanese Unexamined Patent Publication No. 2000-105203, the beam enlargement optical system  16  composed of, for example, a concave lens and a convex lens that are not shown, a lens  14  that reshapes light L 0  emitted from the laser light source  11  into a slit-shaped beam, and a mirror  15 . The illuminative optical system  10  reshapes the light L 0 , which is emitted from the laser light source  11 , into the slit-shaped beam  201 , and irradiates the slit-shaped beam to a slit-shaped area  201 - 1  on the wafer  1 . 
     The inspection system of the present embodiment includes, as an arrangement for illuminating the surface of the wafer  1  with a single-wavelength laser beam at a low angle (low angle of incidence), an arrangement that irradiates the slit-shaped beam  201  (light irradiated to the slit-shaped area  201 - 1  on the wafer and referred to as the slit-shaped beam) to the wafer  1  (substrate to be inspected) on the specimen placement table  34  on a planar basis in a plurality of directions (four directions  220 ,  230 ,  240 , and  250  in  FIG. 2B ) and at a plurality of illuminating angles (angles α, β, and γ in  FIG. 2B ). 
     The reason why illumination light is reshaped into the slit-shaped beam  201  is that an image carried by light that is scattered from a foreign matter or defect under illumination is formed on the detective surfaces of the photodetectors  26  defined by the respective arrays of light receiving elements, and thus detected comprehensively in order to speed up inspection of a foreign matter. 
     Specifically, the θ stage  33  is driven so that the directions, in which the chips  22  constituting the wafer  1  are arrayed, will be parallel to the scanning direction of the X stage  31 - 1  and the scanning direction of the Y stage  31 - 2 . Thus, the orientation of the wafer  1  placed on the placement table  34  is adjusted, and the slit-shaped beam  201  is irradiated to the wafer  1  having the orientation thereof adjusted. 
     The slit-shaped area  201 - 1  on the wafer  1  to which the slit-shaped beam  201  is irradiated is defined by adjusting the ray axis of the beam so that it will be perpendicular to the scanning direction X of the X stage  31 - 1  (the longitudinal direction of the slit-shaped beam  201  irradiated to the wafer  1  will be perpendicular to the scanning direction X of the X stage  31 - 1 ), parallel to the scanning direction Y of the Y stage  31 - 2  (the longitudinal direction of the slit-shaped beam  201  irradiated to the wafer  1  will be parallel to the scanning direction Y of the Y stage  31 - 2 ), and parallel to the direction in which the pixel locations in the photodetectors  26   a  and  26   b  are arrayed. This is advantageous in that when image signals representing the surfaces of chips are compared with each other, the chips can be readily aligned with each other. The slit-shaped beam  201  can be produced by inserting a conically curved lens  14 ,  224 , or  234  shown in  FIG. 3A , a cylindrical lens  255  shown in  FIG. 3B , or a cylindrical lens  244  shown in  FIG. 3C  into a path. 
     For illuminations achieved in the directions  220  and  230  respectively shown in  FIG. 2B , laser light shaped like a slit is irradiated to the slit-shaped area  201 - 1  on the wafer  1  in a direction that is laterally deviated by an angle φ from the Y-axis direction of the wafer and that meets the surface of the wafer  1  at an angle α (in  FIG. 2B , a path along which illumination light propagating in the direction  230  is reflected from the mirror  233 , passes through the cylindrical lens  234 , and reaches the mirror  236  is shown superimposed on a path extending from the mirror  236  to the area  201 - 1  on the wafer  1  to which the slit-shaped beam  201  is irradiated). 
     As an arrangement for realizing the foregoing illuminations, the conically curved lens  14  whose radius of curvature relative to the longitudinal direction varies continuously (corresponding to the lens  224  or  234  shown in  FIG. 2B ) is, as shown in  FIG. 3A , disposed in the path so that the major-axis direction of the slit-shaped beam  201  that is irradiated to the wafer  1  will be parallel to the scanning direction of the Y stage  31 - 2  (the mirrors  225 ,  226 ,  235 , and  236  are excluded from  FIG. 3 ). 
     As for the illuminations achieved in the directions  240  and  250 , since the illuminations are achieved in the same directions as the scanning directions of the X stage  31 - 1  and Y stage  31 - 2  respectively or the directions perpendicular to the scanning directions, the lens  255  shown in  FIG. 3B  (mirror  245  is excluded) or the lens  244  shown in  FIG. 3C  is used to produce the slit-shaped beam  201  so as to illuminate the slit-shaped area  201 - 1  on the wafer (in  FIG. 3B  and  FIG. 3C , the lenses  244  and  255  are shown as cylindrical lenses but are not necessarily cylindrical when a shift of a position on which light irradiated to the wafer  1  is focused has to be compensated). 
     As shown in  FIG. 2A , the mirror  15  (corresponding to the mirror  226  or  236  shown in  FIG. 2B ) and the mirror  205  (corresponding to the mirror  225  or  235  shown in  FIG. 2B ) are mechanically switched in response to a command issued from the overall control unit  50 . Thus, an illuminating angle α can be varied depending on a kind of foreign matter to be detected on the substrate  1  to be inspected. As shown in  FIG. 2C , whatever illuminating angle is designated, the slit-shaped area  201 - 1  on the wafer  1  to which the slit-shaped beam  201  is irradiated covers the direction  203  in which the pixel locations included in the photodetectors  26   a  and  26   b  are arrayed. In whatever directions including directions opposite to the directions  220  and  230  illumination is achieved, the position illuminated by the slit-shaped beam  201  corresponds to the slit-shaped area  201 - 1  on the wafer  1 . 
     Consequently, illumination can be achieved at an angle φ of about 45° with light including rays parallel to the Y direction. In particular, when the slit-shaped beam  201  is composed of rays parallel to the Y direction, diffracted light emanating from circuit patterns whose major group of straight lines is oriented in the X and Y directions is intercepted by the spatial filter  22 . 
     Incidentally, methods of manufacturing the conically curved lens  14  include a method described in, for example, Japanese Unexamined Patent Publication No. 2000-105203. 
     The reason why the slit-shaped beam  201  is irradiated to the slit-shaped area  201 - 1  on the wafer  1  at a plurality of illuminating angles is to detect various kinds of foreign matters present on the surface of the wafer  1 . Specifically, a defective pattern on the substrate  1  to be inspected or a short foreign matter is detected. 
     A more microscopic foreign matter or defect must be detected in a portion of each of chips  202  constituting the wafer  1  in which the same pattern is repeatedly formed at intervals of a relatively narrow pitch in order to realize a memory unit or the like. A method conventionally known for detection of the microscopic foreign matter or defect is such that: relatively intense light is fed to the surface of the wafer  1  at a low angle with respect to the surface of the wafer; and diffracted light emanating from repetitive patterns formed closely at intervals of a relatively narrow pitch is intercepted by the spatial filter  22  in order to detect light reflected or scattered from the foreign matter or defect on the surface of the wafer  1 . As a light source of the relatively intense light to be used to illuminate the surface of the wafer  1  at the low angle, a laser is suitable. 
     However, for detection of a foreign matter or defect in non-repetitive patterns formed to realize a logic unit or the like or loosely repetitive patterns formed at intervals of a relatively coarse pitch, since light scattered from the non-repetitive patterns or the loosely repetitive patterns formed at intervals of a relatively coarse pitch cannot be fully intercepted by the spatial filter, it is hard to extract only the light scattered from the foreign matter or defect from light reflected or scattered from the wafer  1 . For detection of the foreign matter or defect in the non-repetitive patterns formed to realize a logic unit or the like, the wafer  1  should be illuminated in a direction of a relatively high angle. Thus, images of adjoining chips are detected and then compared with each other. However, when an attempt is made to detect a foreign matter or defect on the wafer  1  coated with an optically transparent film using a laser light source, since laser light falls within an extremely narrow range of wavelengths, if the thickness of the transparent film coated over the surface of the wafer varies to exhibit a distribution, the intensity of light reflected from the surface changes due to interference. This disables stable detection of a defect. According to the present embodiment, for detection of a foreign matter or defect in repetitive patterns, a light source whose light falls within a wide wavelength band is used to illuminate the wafer  1  in a direction of a relatively high angle in efforts to avoid the influence of the variation in the thickness of the transparent film coated over the surface of the wafer  1 . 
     The present embodiment has an arrangement for realizing high-angle illumination that is achieved with light falling within a wide wavelength band and low-angle illumination that is achieved with single-wavelength light. 
     To begin with, a low-angle illumination optical system will be described in conjunction with  FIG. 2A  to  FIG. 3C .  FIG. 2A  is an explanatory diagram concerning the principles of an arrangement that is the low-angle illumination optical system. With an increase in an illuminating angle α for low-angle illumination, an amount of light reflected and diffracted by circuit patterns increases, and a signal-to-noise ratio decreases. Therefore, an experimentally obtained optimal value is adopted. For example, for detection of a low foreign matter on the surface of a wafer, the illuminating angle α should be small, for example, should range from 1° to 10°, or preferably, from 1° to 5°. 
     As for an illuminating direction φ for low-angle illumination, for example, at a wiring step, the direction wiring patterns formed on a wafer and the illuminating direction are aligned with each other. This makes it easier to detect a foreign matter in wiring. Moreover, when circuit patterns on a wafer are not wiring patterns but consist of contact holes and capacitors, the circuit patterns have no particular directivity. Therefore, a chip containing the circuit patterns should be illuminated in a direction of about 45°. Talking of changing of illuminating angles, in the case of low-angle illumination, the two mirrors  15  and  205  whose angles are different from each other are, for example, as shown in  FIG. 2A , switched. Otherwise, the angle of the mirror  15  (or 205) may be changed with the X direction (perpendicular to the sheet of paper of the drawing) as an axis of rotation using a rotating means that is not shown. At this time, the mirror  15  is moved even in the Z direction so that the ray axis of the slit-shaped beam  201  will be aligned on a wafer with the ray axis of light detected by the detective optical system. Moreover, the lens  14  is also moved in the Z direction so that the slit-shaped beam  201  will have a minimum diameter on the ray axis of the light detected by the detective optical system. 
     Next, a method for changing illuminating directions will be described in conjunction with  FIG. 2B . A branching optical element  218  shown in  FIG. 2B  is composed of a mirror, a prism, and others. A driving means that is not shown is used to move the branching optical element  218  in the Y direction, whereby laser light L 0  emitted from the laser light source  11  is transmitted or reflected to propagate in any of three directions. Laser light L 1  transmitted by the branching optical element  218  is branched into transmitted light and reflected light by a half prism  221 . For example, after the transmitted light passes through a wave plate  236 , it is reflected by a mirror  236  via a mirror  231 , a beam diameter correction optical system  232 , a mirror  233 , and a conically curved lens  234 . Consequently, the slit-shaped beam  201  enters the wafer  1  in the direction  230 . Incidentally, the mirror  235  is disposed so that it can be inserted into or withdrawn from the space between the conically curved lens  234  and mirror  236 , whereby an angle α of incidence at which the slit-shaped beam  201  formed by the conically curved lens  234  falls on the slit-shaped area  201 - 1  on the wafer can be changed from one to another. 
     On the other hand, light reflected from the half prism  221  is handled by an optical element having the same ability as the foregoing ability so that the slit-shaped beam  201  will be irradiated to the wafer  1  in the direction  220 . Even in this path, a mirror  225  is disposed so that it can be inserted into or withdrawn from the space between a conically curved lens  224  and a mirror  226 . Thus, an angle α of incidence at which the slit-shaped beam  201  formed by the conically curved lens  224  falls on the slit-shaped area  201 - 1  on the wafer can be changed from one to another. Incidentally, beam diameter correction optical systems  222  and  232  adjust the diameters of laser light waves incident on the conically curved lenses  234  and  224  respectively so that the slit-shaped beams  201  to be irradiated to the wafer  1  will have the same size. Moreover, if a mirror  260  is substituted for the half prism  221 , illumination is achieved in the direction  220  alone. If neither the half prism  221  nor the mirror  260  is used, illumination is achieved in the direction  230  alone. Moreover, a wave plate  226  or  236  located behind the half prism  221  may be used to designate a direction of polarization of laser light to be irradiated so as to produce, for example, p-polarized light or s-polarized light alone. 
     After laser light L 2  reflected from the branching optical element  218  passes through the beam diameter correction optical system  241 , it is reflected from the mirrors  242  and  243 , transmitted by a cylindrical lens  244  (see  FIG. 3C ), and reflected by a mirror  245 . Consequently, a beam reshaped like a slit by the cylindrical lens  244  is irradiated to the slit-shaped area  201 - 1  on the wafer  1  in the direction  240 . 
     On the other hand, laser light L 3  is reflected by a mirror  251  in the same manner as light L 2  is reflected by the foregoing optical element disposed on the path, and transmitted by a beam diameter correction optical system  252 . Thereafter, the laser light is reflected from mirrors  253  and  254 , transmitted by a cylindrical lens  255 , and reflected by a mirror  256 . Consequently, the slit-shaped beam  201  is irradiated to the wafer  1  in the direction  250 . 
     As for the illuminating directions  240  and  250 , if many wiring patterns formed on a wafer are parallel to the X or Y direction, the directions of illumination can be aligned with the X or Y direction at, for example, a wiring step. This is advantageous in that a foreign matter in wiring can be readily detected. 
     As the laser light source  11 , a high-power YAG laser whose laser light contains a second harmonic having a wavelength of 532 nm is adopted. However, the wavelength need not always be 532 nm, and the fourth harmonic of the laser light having a wavelength of 266 nm may be utilized. Moreover, an ultraviolet, far-ultraviolet, or vacuum-ultraviolet laser, an Argon laser, a nitrogen laser, a He—Cd laser, an eximer laser, or a semiconductor layer may be adopted as a light source. 
     In general, when the wavelength of laser light is shortened, the resolution of a detected image improves. This permits high-sensitivity inspection. 
     On the other hand,  FIG. 4  shows in detail illumination to be achieved using a wide wavelength band, though the illumination light source  12  to be used for the illumination is illustrated in  FIG. 1 . In  FIG. 4A  to  FIG. 4C , light falling within a wide wavelength band and emanating from the light source  12  is, as shown in  FIG. 4A , branched into three paths by half-silvered mirrors  1801  and  1802  included in a high-angle illumination optical system  18 . After the light waves have their paths angled by mirrors  1803  to  1811 , the light waves are, as shown in  FIG. 4B , converged on a group of condenser lenses  1813 . Consequently, the substrate  1  to be inspected is illuminated in eight directions, which are equiangular directions with θ=45° between adjoining ones, by a group of mirrors  1824  to  1831 . The transmittances of the mirrors  1801  to  1805 ,  1807 ,  1809 , and  1810  disposed along the paths are determined so that the intensities of illuminations in the respective directions will be equal to one another on the substrate  1  to be inspected. 
       FIG. 5A  and  FIG. 5B  show other examples of high-angle illumination. 
       FIG. 5A  shows an arrangement in which: a wavelength selection filter  2002  selects light, which falls within a specific range of wavelengths, from light that is emitted from a wide-band lamp light source  12  and that is transmitted by a lens  2001 ; after transmitted by a lens  2003 , the selected light is routed to a half-silvered mirror  2005 , which is interposed between the objective lens  21  and the spatial filter  22  or between the spatial filer  22  and the image formation lens  23 , via collimator lenses  2004 , and then irradiated using spherical mirrors  2006  so that the numerical aperture of the objective lens  21  will not be impaired.  FIG. 5B  shows an arrangement for irradiating light, which is emitted from the wide-band lamp light source  12 , as it is without selecting a specific wavelength. For detection of a foreign matter in wiring at a wiring step or detection of a defective pattern, an illuminating angle η should be increased. However, in consideration of the relationship between a pattern and a signal-to-noise ratio for a signal acquired from a foreign matter, the illuminating angle η should range from 40° to 60°, or preferably, from 45° to 55°. Moreover, if a step of handling an object of inspection is associated with a kind of foreign matter to be detected, whatever illuminating angle is selected may be designated in an inspection plan. 
     Moreover, for detection of all foreign matters or defective patterns on the surface of a wafer without leaving any foreign matter or defective pattern uninspected, the illuminating angle for high-angle illumination described in conjunction with  FIG. 4  or  FIG. 5  may be set to an intermediate range of the aforesaid range of values, that is, a range from 5° to 45°. 
       FIG. 6A  and  FIG. 6B  show still other examples of high-angle illumination. As shown in  FIG. 6A , light emitted from the light source  12  is propagated through a wavelength selection filter  2101 , split into eight light waves by optical fibers  2102  to  2109 , and converged on lenses  2112  to  2119  attached to the emission ends of the respective optical fibers. The light waves are then, as shown in  FIG. 6B , reflected from a group of mirrors  2120  in order to change their paths. Consequently, high-angle illumination is performed on the wafer  1 . 
     Next, the detective optical system  20  will be described below. 
     The detective optical system  20  is configured so that light reflected and diffracted by the substrate  1  to be inspected such as a wafer is detected by the photodetectors  26   a  and  26   b , which are realized with TDI image sensors or the like, via the objective lens  21 , spatial filter  22 , image formation lens (variable-power image formation optical system)  23 , optical filter  25  composed of a density filter and a sheet polarizer, and beam splitter  29 . When the TDI sensors are adopted as the photodetectors  26   a  and  26   b , the TDI sensors each having a plurality of output taps provide a plurality of signals concurrently. The signal processing system  40  manipulates the plurality of signals concurrently using a plurality of processing circuits or a plurality of pieces of processing software. Consequently, a defect can be detected quickly. 
     The spatial filter  22  has the ability to intercept a Fourier-transform image carried by light reflected and diffracted by repetitive patterns on the wafer  1  and pass light scattered from a defect or foreign matter. The spatial filter  22  is disposed in an area where it helps the objective lens  21  offer the spatial frequency, that is, at the position of an image plane on which a Fourier transform is formed (corresponding to an exit pupil). The spatial filter  22  exhibits an optical characteristic of intercepting a specific wavelength alone and transmitting the other wavelengths. Specifically, the spatial filter  22  intercepts light, which is diffracted by repetitive patterns formed in a memory unit or any other area and is derived from illumination light irradiated to the substrate  1  to be inspected, and passes light scattered from a defect or foreign matter. Moreover, the spatial filter  22  passes light diffracted by non-repetitive patterns on the surface of the substrate  1  to be inspected that is illuminated by the wide-band illumination light source  12 . Light passing through the spatial filter  22  is separated into different wavelengths by the beam splitter  29 . A wavelength identical to the wavelength of light emitted from the laser light source  11  is reflected to reach the photodetector  26   a , and the other wavelengths are transmitted to reach the photodetector  26   b . This makes it possible to avoid the saturation of each photodetector in terms of an amount of received light which is caused by light diffracted by the non-repetitive patterns. 
     Incidentally, a pupil viewing optical system  70  that is composed of a mirror  90  which can withdraw in the Y direction during inspection, a projection lens  91 , and a TV camera  92  and that is disposed along the optical axis of the detective optical system  20  is used to image bright points  502  (drawn with filled circles in  FIG. 8A ) in an image, which is carried by light reflected and diffracted by repetitive patterns and is viewed at the position of the image plane of a Fourier transform, within a field of view  501  offered by the pupil viewing optical system  70 . A sheet interceptor  503  including a rectangular interceptor like the one shown in  FIG. 8B  is disposed at the position of the image plane of a Fourier transform. Both ends of the sheet interceptor  503  are, as shown in  FIG. 9D , fixed to springs  715  and  716 . The resultant sheet interceptor  503  is fixed to an arm  710  that includes, as shown in  FIG. 9A  and  FIG. 9B , a motor  725 , and a pair of members  710   a  and  710   b  capable of being opened or closed in the X direction by means of a feed screw  720  and a slide guide  722 . A spacing Wx between adjoining ones of slats included in the sheet interceptor  503  is mechanically varied, and the entire sheet interceptor is moved along a guide  728  by means of a motor  726  and a feed screw  737  (in directions Ox in  FIG. 8B ). Thus, the rectangular interceptor is adjusted so that the spacing between adjoining slats will be equal to a pitch Px between adjoining ones of the bright points  502  in the reflected and diffracted light image. Specifically, the sheet interceptor  503  is, as shown in  FIG. 8B , adjusted for fear the bright points in the image of the light reflected and diffracted by the repetitive patterns, which is viewed at the position of the image plane of a Fourier transform, may come out of the sheet interceptor  503 . 
     As mentioned above, the spacing Wx between adjoining ones of the slats included in the sheet interceptor  503  is determined based on an actual image of patterns observed through the pupil viewing optical system  70 . Moreover, design data representing patterns may be used to create a Fourier-transform image, and the spacing may be determined based on the image. Moreover, a sheet interceptor  503 ′ whose interceptive width is different from that of the sheet interceptor  503  may be attached to the arm  710 . In this case, if a table  740  bearing the entire arm  710  is moved in the Y direction along a guide  732  by means of a motor  735  and a feed screw  730  in order to switch the sheet interceptors, a change in the size of bright points in a reflected and diffracted light image can be coped with. The switching is performed in response to a command sent from the overall control unit  50  after a signal acquired by the TV camera  92  is manipulated by the signal processing system  95 . Incidentally, the sheet interceptor  503  may not be employed. Alternatively, based on an image signal produced by the TV camera  92 , an interceptor may be formed on a transparent substrate included in, for example, a liquid crystal display device, with black and white reversed, and substituted for the sheet interceptor  503 . 
       FIG. 8C  shows an image (bright points  507 ) that is carried by light reflected and diffracted by repetitive patterns and that is viewed at the position of the image plane of a Fourier transform within a field of view  501  offered by the pupil viewing optical system  70  when the wafer  1  is illuminated simultaneously in different directions. When the bright points  507  are laterally widened, a two-dimensional interceptive pattern that is, as shown in  FIG. 8D , a combination of a lengthwise interceptive pattern  503  and a sideways interceptive pattern  504  is used to intercept the bright points  507  in the image carried by the light reflected and diffracted by the repetitive patterns formed on the wafer  1 . 
     A sheet interceptor  504  having, as shown in  FIG. 9C , the same structure as the one described in conjunction with  FIG. 9A  and  FIG. 9B  is included and superimposed on the sheet interceptor  503  so that the slats extending in the X and Y directions will intersect one another. In this case, the sheet interceptor  503  is placed on the bottom of the arm  710  including the pair of members that can be opened or closed, while the sheet interceptor  504  is placed on the top of the arm  710  so that it will jut out. Consequently, the interceptors will not be defocused in the Z direction on the plane containing the pupil of the objective lens  21 . 
     Owing to the above arrangement, by adjusting the spacing Wx in the X direction between adjoining ones of the slats included in the sheet interceptor  503 , the spacing Wy in the Y direction between adjoining ones of the slats included in the sheet interceptor  504 , and the phases Ox and Oy of the sheet interceptor assembly, the spacings in the respective directions between adjoining ones of the slats included in the two-dimensional interceptive pattern made by combining the lengthwise interceptive pattern  503  and the sideways interceptive pattern  504  can be, as shown in  FIG. 8D , adjusted. 
     Consequently, as shown in  FIG. 10 , even when a plurality of memory units A to D exists in a chip, the spacings Wx and Wy between slats included in the interceptor included in the spatial filter are preserved during designation of conditions prior to inspection, and then designated immediately before the position of an area to be inspected is focused on. Thus, a microscopic defect can be detected highly sensitively. 
     In the present embodiment, the detective optical system  20  is designed to detect light, which is reflected or scattered from the wafer  1  after the wafer  1  is illuminated by irradiating laser light, which is emitted from the laser light source  11 , via the low-angle illumination system as described in conjunction with  FIG. 2A  to  FIG. 3C , separately from light that is reflected or scattered from the wafer  1  illuminated with light, which is emitted from the wide-band light source  12  and falls within a wide wavelength band, via the high-angle illumination system as described in conjunction with  FIG. 4A  to  FIG. 6B . Specifically, the beam splitter  29  shown in  FIG. 1  reflects light that is reflected or scattered from the wafer  1  illuminated with light irradiated via the low-angle illumination system. The photodetector  26   a  detects the reflected light. The beam splitter  29  transmits light that is reflected or scattered from the wafer  1  illuminated with light irradiated via the high-angle illumination system, and the photodetector  26   b  detects the transmitted light. 
       FIG. 11A  shows the detective optical system in detail. Herein, reference numeral S 0  denotes light that is reflected or scattered from the wafer  1  and transmitted by the objective lens  21  and image formation lens  23 . In the light, light reflected or scattered from the wafer  1  after irradiated via the low-angle illumination system, and light reflected or scattered from the wafer  1  after irradiated via the high-angle illumination system are mixed. The surface of the beam splitter  29  is coated with a thin film  2901  having a property of reflecting light whose wavelength is identical to that of laser light emitted from the laser light source  11 . Among the components of the light S 0  reflected or scattered from the wafer  1 , a reflected or scattered light component S 1  of laser light emitted from the laser light source  11  is reflected from the beam splitter  29  and then routed to the photodetector  26   a . The detecting surface of the photodetector  26   a  (not shown) is located at the position of the image plane of an image formed by the image formation lens  23 . The photodetector  26   a  detects an image of light reflected or scattered from the wafer  1  and derived from low-angle illumination achieved by the laser. On the other hand, the thin film  2901  coated over the surface of the beam splitter  29  transmits light having wavelengths other than the same wavelength as the wavelength of laser light. Therefore, a light component S 2  reflected or scattered from the wafer  1  and derived from high-angle illumination achieved with light falling within a wide wavelength band enters the photodetector  26   b  located at the position of the image plane of an image formed by the image formation lens  23 . The photodetector  26   b  detects a reflected or scattered light image derived from high-angle illumination achieved with light emitted from the wide-band light source  12 . 
     Assuming that the light S 0  reflected or scattered from the wafer  1  is not separated into the components S 1  and S 2  but detected as it is by the photodetector  26   a , the detective signal has a defect signal  950  buried in noises as shown in  FIG. 11B . On the other hand, when the component S 1  is separated from the light S 0  reflected or scattered from the wafer  1  and then detected by the photodetector  26   a  as it is in the present embodiment, the detective signal has a defect signal  960  distinguished as shown in  FIG. 11C . Thus, the defect signal can be detected. 
     On the other hand, light emitted from the wide-band light source  12  for high-angle illumination is set to a lower intensity than light emitted from the laser light source  11  for low-angle illumination is. The photodetector  26   b  that detects light reflected or scattered from the wafer  1  and derived from low-angle illumination achieved by the wide-band light source  12  is set to a higher sensitivity than the photodetector  26   a  is. At this time, if the light component S 1  were not separated from the light S 0  reflected or scattered from the wafer  1  but were detected by the photodetector  26   b , the photodetector  26   b  might be saturated due to the component S 1 . However, in the present embodiment, since the photodetector  26   b  detects the light S 2  that remains intact with the component S 1  separated from the light S 0  reflected or scattered from the wafer  1 , the photodetector will not be saturated but can stably detect light. 
     In the present embodiment, relatively high-luminance light reflected or scattered from a foreign matter on the surface of the wafer  1  and derived from low-angle illumination achieved with relatively high-luminance laser light emitted from the laser light source  11  is detected separately from light that is reflected or scattered from the surface of the wafer  1  and that is derived from high-angle illumination achieved with wide-band light emitted from the wide-band light source  12 . Therefore, low-angle illumination with laser light and high-angle illumination with wide-band light can be achieved simultaneously. Consequently, the whole of the wafer  1  can be inspected during one inspection. Namely, both an area on the wafer  1  having repetitive patterns formed therein at intervals of a relatively small pitch (for example, patterns in a memory unit) and an area having patterns formed therein at intervals of a relatively large pitch (for example, non-memory patterns in a logic unit) can be continuously inspected to detect a defect without the necessity of switching optical systems. 
     The inspection system of the present embodiment supports both a mode in which foreign matter inspection is performed quickly and a mode in which high-sensitivity inspection is performed slowly. For an object of inspection or an area in which circuit patterns are formed at a high density, the power of the detective optical system is raised in order to acquire a high-resolution image signal. This permits highly sensitive inspection. Moreover, for an object of inspection or an area in which circuit patterns are formed at a low density, the power of the detective optical system is lowered in order to perform inspection quickly with high sensitivity maintained. 
     Consequently, the size of a foreign matter that should be detected and the size of a defect-detected pixel can be optimized. Noises caused by anything other than a foreign matter are removed, and only light scattered from the foreign matter can be detected efficiently. Namely, in the inspection system in accordance with the present embodiment, the power of the detective optical system  20  located above the wafer  1  can be varied using a simple arrangement. 
     Next, an arrangement for varying the power of the detective optical system and movements to be performed will be described in conjunction with  FIG. 7 . 
     The power of the detective optical system is varied in response to a command issued from the overall control unit  50 . The image formation lens  23  includes movable lenses  401 ,  402 , and  403  and a moving mechanism  404 . When the power is varied, the magnification of a wafer surface image formed on the photodetector  26   a  or  26   b  can be varied without the necessity of changing the positions of the objective lens  21  and spatial filter  22  in an optical-axis direction. In other words, the relative positions between the substrate  1  to be inspected and the photodetector  26   a  or  26   b  need not be changed even at the time of varying the power. The moving mechanism  404  having a simple structure can be used to vary the power. Furthermore, since the size of the image plane of a Fourier transform remains unchanged, the spatial filter  22  need not be modified. 
     Assuming that f 1  denotes the focal length of the objective lens  21  and f 2  denotes the focal length of the image formation lens  23 , the power M of the detective optical system  20  is calculated according to formula 1 presented below. 
         M=f   2   /f   1   (1)
 
     In order to realize the power M of the variable-power detective optical system  20 , since f 1  denotes a fixed value, the movable lenses are moved to positions permitting f 2  to assume the product of M by f 1 . 
       FIG. 7  shows the configuration of the image formation lens  23  including the movable lenses  401  to  403  and the moving mechanism  404 , and shows an arrangement for moving the movable lenses  401 ,  402 , and  403  to specific positions so as to thus align them. The movable lens  401  is held by a lens holder  410 , and the lens holder  410  moves in optical-axis directions along a linear guide  450  along with the rotation of a ball screw  412  driven by a motor  411 . Likewise, the movable lenses  402  and  403  held by lens holders  420  and  430  respectively can move independently of each other in the optical-axis directions along the linear guide  450  along with the rotations of ball screws  422  and  432  respectively driven by motors  421  and  431  respectively. 
     Specifically, movable members  415 ,  425 , and  435  included in alignment sensors are attached to the distal ends of the lens holders  410 ,  420 , and  430  respectively that hold the movable lenses  401 ,  402 , and  403  respectively. Detectors  416 ,  426 , and  436  included in the alignment sensors are disposed at positions at which the movable lenses  401 ,  402 , and  403  respectively are halted. The motors  411 ,  421 , and  431  are driven in order to move the lens holders in the optical-axis directions. The alignment sensors  416 ,  426 , and  436  disposed at positions permitting a desired power detect the movable members  415 ,  425 , and  435  respectively so as to align them. Alignment sensors  417  and  418  serve as limit sensors for detecting the upper and lower limits in the optical-axis directions of a movable range of the movable lens  401 . Likewise, limit sensors  427  and  428  and limit sensors  437  and  438  are included for the movable lenses  402  and  403  respectively. As the alignment sensor, an optical or magnetic sensor is conceivable. 
     The foregoing movements are performed in response to a command issued from the overall control unit  50 . For example, when the circuit patterns formed on the substrate  1  to be inspected are highly dense, a high-power high-sensitivity inspection mode is designated. When the circuit patterns are less dense, quick inspection is performed with a low power. Thus, the power is appropriately designated so that many microscopic defects can be detected according to information on the surface of the substrate  1  to be inspected which is placed on the stages or according to a step included in a manufacturing process. 
     Next, a description will be made of a conditions-for-inspection designation sequence to be followed by the foreign matter or defect inspection system in accordance with the present invention. 
       FIG. 12  is a flowchart describing designation of conditions for inspection. The wafer  1  is placed on the table  34  by a loader and immobilized by performing vacuum absorption (S 2800 ). Chip layout information, that is, information on a chip size of a wafer or presence or absence of a chip in the wafer is designated (S 2801 ). Thereafter, the entire wafer  1  is rotated so that the directions of arrays of chips  202  in the wafer  1  will run parallel to the edges of the photodetectors  26  (a rotation error is nearly nullified) (S 2802 ). Thereafter, for inspection of each area on a wafer with optimal sensitivity, an inspective area is designated and detective sensitivity for the inspective area are designated (S 2803 ). Thereafter, optical conditions such as a direction of illumination light to be irradiated to the wafer, an angle to be selected, and a power to be selected for the variable-power detective optical system  20  are designated (S 2804 ). An optical filter is designated (S 2805 ). An amount of light to be detected is designated (S 2806 ). The conditions for signal processing are designated (S 2807 ). Inspection is then initiated (S 2808 ). 
     Next, manipulation to be performed on a detective signal at an inspection step (S 2808 ) will be described below. Signals sent from the photodetectors  26  having received light reflected and diffracted by the surface of the wafer  1  are manipulated by the signal processing system  40 .  FIG. 13  shows the configuration of the signal processing system  40 . Referring to  FIG. 13 , the signal processing system  40  includes processing circuits  40   a ,  40   b , and  95  associated with the photodetectors  26   a ,  26   b , and  92 . When the number of detectors is increased, the number of processing circuits is increased accordingly. 
     The configuration of the processing circuit  40   a  and the actions to be performed thereby will be described below. The processing circuit  40   a  includes: an A/D converter  1301  that receives a detective signal from the detector  26   a ; a data memory block  1302  in which a detective image signal f(i,j) resulting from the A/D conversion is stored; a threshold calculation block  1303  that calculates a threshold on the basis of the detective image signal; foreign matter detection blocks  1304   a  to  1304   n  each of which includes as circuits, which detect a foreign matter at every merger of pixels on the basis of the detective image signal  1410  read from the data memory block  1302  and threshold image signals (Th(H), Th(Hm), Th(Lm), Th(L)) provided by the threshold calculation block  1303 , pixel merger circuits  1305   a  and  1306   a , a foreign matter detection circuit  1307   a , and an inspective area handling circuit  1308   a ; a characteristic quantity calculation circuit  1309  that calculates a characteristic quantity such as an amount of scattered light obtained by detecting a defect under low-angle illumination, an amount of scattered light obtained by detecting a defect under high-angle illumination, or the number of detected pixels implying a degree of spread of a defect; and an integration block  1310  that classifies defects, which include small and large foreign matters on a semiconductor wafer, a defective pattern, and a micro-scratch, on the basis of the characteristic quantity calculated for each merger by the characteristic quantity calculation block  1309 . 
     The foreign matter detection blocks  1304   a  to  1304   n  have the pixel merger circuits  1305   a  to  1305   n  and  1306  to  1306   n , the foreign matter detection circuits  1307   a  to  1307   n , and the inspective area handling circuits  1308   a  to  1308   n  respectively associated with respective merger operators that handle pixels the number of which corresponds to a product of, for example, 1×1, 3×3, 5×5, etc., or n×n. 
     A signal produced by the photodetector  26   a  is digitized by the A/D converter  1301 . A detective image signal f(i,j)  1410  is stored in the data memory block  1302  and transferred to the threshold calculation block  1303 . The threshold calculation block  1303  calculates a threshold image Th(i,j)  1420  to be used to detect a foreign matter. The foreign matter detection circuit  1307  detects a foreign matter on the basis of signals which the pixel merger circuits  1305  and  1306  have manipulated according to values produced by merger operators of an associated type. 
     The inspective area handling block  1308  manipulates a detected foreign matter signal and a threshold image in terms of a place of detection. Concurrently, based on signals produced by the pixel merger circuits  1305   a  to  1305   n  and  1306   a  to  1306   n , the foreign matter detection circuits  1307   a  to  1307   n , and the inspective area handling blocks  1308   a  to  1308   n  included in the respective foreign matter detection blocks  1304   a  to  1304   n  associated with the respective types of merger operators, the characteristic quantity calculation circuit  1309  calculates a characteristic quantity (for example, an amount of light scattered under high-angle illumination, an amount of light scattered under low-angle illumination, or the number of defect-detected pixels). The integration block  1310  integrates the foreign matter signal with the characteristic quantity, and transmits the resultant data to the overall control unit  50 . 
     To be more specific, the ND converter  1301  is a circuit having the ability to convert an analog signal produced by the photodetector  26  into a digital signal. The number of bits to be converted preferably ranges from 8 to 12 bits. If the number of bits is small, a resolution resulting from signal processing is low. This makes it hard to detect feeble light. On the other hand, if the number of bits is large, an expensive ND converter is needed. This leads to the demerit that the price of the system gets high. The data memory block  1302  is a circuit in which the analog-to-digital converted signal is stored. 
     Next, the pixel merger circuits  1305  and  1306  will be described in conjunction with  FIG. 14 . 
     The pixel merger circuits  1305   a  to  1305   n  and  1306   a  to  1306   n  include mutually different merger operators  1504 . 
     The merger operators  1504  are facilities each of which merges detective image signals f(i,j)  1410  that are read from the data memory block  1302  and that represent respective pixels arrayed in n rows and n columns, and each of which merges difference threshold image signals  1420 , each of which includes a detective threshold image signal Th(H), a detective threshold image signal Th(L), an inspective threshold image signal Th(Hm), and an inspective threshold image signal Th(Lm) that are produced by the threshold calculation block  1303 , concerning the respective pixels arrayed in n rows and n columns. The merger operators  1504  are, for example, circuits each of which produces mean values relevant to the pixels arrayed in n rows and n columns. 
     The pixel merger circuits  1305   a  and  1306   a  include merger operators each of which merges a value relevant to, for example, a pixel arrayed in one row and one column. The pixel merger circuits  1305   b  and  1306   b  include merger operators each of which merges values relevant to pixels arrayed in three rows and three columns. The pixel merger circuits  1305   c  and  1306   c  include merger operators each of which merges values relevant to pixels arrayed in five rows and five columns. The pixel merger circuits  1305   n  and  1306   n  include merger operators each of which merges values relevant to pixels arrayed in n rows and n columns. The merger operators that merge a value relevant to a pixel arrayed in one row and one column provide input signals  1410  and  1420  respectively as they are. 
     The threshold image signal includes, as mentioned above, four image signals (Th(H), Th(Hm), Th(Lm), and Th(L)). Therefore, each of the pixel merger circuits  1306   a  to  1306   n  needs four merger operators Op. Consequently, the pixel merger circuits  1305   a  to  1305   n  transmit merged detective image signals  431   a  to  431   n  that are the results of merger of detective image signals performed by the merger operators  1504 . On the other hand, the pixel merger circuits  1306   a  to  1306   n  transmit merged threshold image signals  441   a  ( 441   a   1  to  441   a   4 ) to  441   n  ( 441   n   1  to  441   n   4 ) that are the results of merger of four sets of threshold image signals (Th(H), Th(Hm), Th(Lm), Th(L)) performed by the merger operators Op 1  to Opn. The merger operators included in the respective pixel merger circuits  1306   a  to  1306   n  are identical to one another. 
     The advantage of merger of values relevant to pixels will be described below. Foreign matter inspection should detect not only a microscopic foreign matter but also a large thin-film-like foreign matter that spreads over a range of several micrometers wide. However, a detective image signal acquired from the thin-film-like foreign matter is not always intense enough. Therefore, a detective image signal representing a pixel exhibits a low signal-to-noise ratio and may therefore be left unfound. Therefore, every set of pixels arrayed in n rows and n columns of which size corresponds to the size of the thin-film foreign matter is cut out and convoluted to other set of pixels, whereby the signal-to-noise ratio is improved. 
     Next, the inspective area handling blocks  1308   a  to  1308   n  will be described below. 
     The inspective area handling blocks  1308   a  to  1308   n  are employed in a case where data representing an area (including an area within a chip) that need not be inspected is removed from a foreign matter or defect detective signal acquired from a specific chip by each of the foreign matter detection circuits  1307   a  to  1307   n , in a case where a detective sensitivity has to be changed area by area (including an area within a chip), or in a case where an area to be inspected is selected. 
     Regarding the inspective area handling blocks  1308   a  to  1308   n , for example, when the detective sensitivity for an area on the substrate  1  to be inspected may be low, a threshold for the area calculated by a threshold calculator  1411  included in the threshold calculation block  1303  may be set to a large value. Otherwise, data representing a foreign matter in an area that should be inspected may be extracted from data items representing foreign matters, which are produced by each of the foreign matter detection circuits  1307   a  to  1307   n , on the basis of coordinates representing the position of the foreign matter. 
     An area for which detective sensitivity may be low is, for example, an area on the substrate  1  to be inspected where the density of circuit patterns is low. The merit of lowering the detective sensitivity is that the number of foreign matters to be detected is efficiently decreased. A high-sensitivity inspection system may detect as many as several ten thousands of foreign matters. At this time, what is most significant is a foreign matter in an area in which circuit patters exist. Taking measures against the significant foreign matter provides a shortcut to improvement in a yield of device manufacture. 
     However, when the whole of the substrate  1  to be inspected is inspected with the same sensitivity, since a significant foreign matter and an insignificant foreign matter coexist, the significant foreign matter cannot be readily extracted. Therefore, the inspective area handling blocks  1308   a  to  1308   n  are used to lower the detective sensitivity for an area, of which foreign matter do not adversely affect a yield and in which no circuit pattern exists, on the basis of CAD information on each chip or threshold map information. Thus, the significant foreign matter can be efficiently extracted. However, a method of extracting a foreign matter is not limited to the method of changing detective sensitivity. Alternatively, foreign matters may be classified as described later in order to extract a significant foreign matter or the significant foreign matter may be extracted based on the sizes of foreign matters. 
     Next, the characteristic quantity calculation circuit  1309  will be described below. 
     What is referred to as a characteristic quantity is a value representing the characteristic of a detected foreign matter or defect. The characteristic quantity calculation circuit  1309  is a processing circuit for calculating the characteristic quantity. The characteristic quantity is, for example, an amount of light reflected and diffracted by a foreign matter or defect and derived from high-angle illumination or low-angle illumination (an amount of scattered light) (Dh,Dl), the number of defect-detected pixels, the shape of a foreign matter-detected area, the direction of a principal axis of inertia, a place on a wafer where a foreign matter is detected, a type of circuit patterns on a substrate, or a threshold for detection of a foreign matter. 
     Next, the integration block  1310  will be described below. 
     The integration block  1310  has the ability to integrate the results of foreign matter detections concurrently performed by the pixel merger circuits  1305  and  1306  respectively or integrate a characteristic quantity calculated by the characteristic quantity calculation circuit  1309  with the results of foreign matter detection (positional information on a foreign matter or defect) and to transmit the result of the integration to the overall control unit  50 . The integration of the results of inspection should preferably be performed by a personal computer or the like so that the contents of processing can be easily modified. 
     The processing circuit  40   b  performs the same processing as the foregoing processing circuit  40   a  does. An image signal produced by the photodetector  26   b  that has detected light reflected or scattered under high-angle illumination is manipulated by the components ranging from an A/D converter  2301  to a results-of-inspection integration block  2310  in order to detect a foreign matter or defect. The characteristic quantity concerning the foreign matter or defect is then calculated. 
     The overall control unit  50  adds up (ORs) foreign matter or defect detective signals, which have been manipulated by the results-of-inspection integration blocks  1310  and  2310  so as to specify the position and characteristic quantity of a foreign matter or defect existing on the wafer  1 . Information on the specified foreign matter or defect is displayed on the screen of the display means  52  in the form of a map representing the surface of a wafer or a histogram one of whose axes indicate sizes. 
     On the other hand, an image signal representing bright points in an image that is carried by light reflected and diffracted by repetitive patterns formed on the wafer  1  and that is picked up by the TV camera  92  and viewed at the position of the image plane of a Fourier transform in the detective optical system  20  is transferred to the signal processing circuit  95 , converted into a digital signal by an ND converter  3301 , and then manipulated as image data by an image data processor  3320 . The signal manipulated as image data by the image data processor  3320  is transferred to a pattern pitch arithmetic block  3330 . A pitch Px between adjoining ones of bright points  502  in a reflected and diffracted light image is calculated. Data representing the pitch Px of the bright points  502  and the image data are transferred to the overall control unit  50 , and then transmitted as a signal, with which the pitches Wx and Wy between adjoining ones of slats included in the interceptive patterns  503  and  504  incorporated in the spatial filter  22  are controlled, to a spatial filter control unit  27 . 
     In the present embodiment, while both low-angle illumination based on laser light and high-angle illumination based on wide-band light are performed concurrently, the wafer  1  is inspected for foreign matters or defects. An image carried by light that is a component of light reflected or scattered from the surface of the wafer  1  as a result of the low-angle illumination based on laser light and that is not intercepted by the spatial filter  22  but reflected from the beam splitter  29  is detected by the photodetector  26   a  and transferred to a processing circuit. A diffraction pattern produced by light that is a component of light reflected or scattered from the surface of the wafer  1  after light falls on the wafer at a low angle and that is scattered from repetitive patterns formed on the wafer  1  at intervals of a relatively small pitch is intercepted by the spatial filter and then detected. Moreover, multi-wavelength light emitted from the lamp light source is caused to fall on the surface of the wafer  1  at a relatively high angle. Light reflected or scattered from the surface of the wafer  1  is detected separately from light scattered under illumination based on single-wavelength laser light. Consequently, during one inspection, both an area on the wafer  1  having repetitive patterns formed therein at intervals of a relatively small pitch (for example, patterns in a memory unit) and an area having patterns formed therein at intervals of a relatively large pitch (for example, non-memory patterns in a logic unit or the like) can be inspected in order to detect defects. 
     Second Embodiment 
     During foreign matter inspection, even a multilayer wafer having a transparent film (for example, an oxide film) coated over the surface thereof must be inspected. The multilayer wafer is manufactured by repeating a step of forming patterns on the transparent film. There is an increasing need for detection of only foreign matters on the surface of an oxide film during inspection of a wafer having the oxide films formed therein. Fundamentally, by decreasing an illuminating angle α, an adverse effect of pattern-diffracted light or light reflected from a substrate can be suppressed. However, the decrease in the illuminating angle α poses a problem in that: light that is a regularly reflected component of illumination light, that is, forward scattered light occupies a majority of light scattered from a foreign matter; an amount of scattered light incident on a detective optical system located above gets smaller; and the foreign matter cannot therefore be detected stably. 
     In the second embodiment, a system shown in  FIG. 15  is used to detect light, which is reflected or scattered from the wafer  1  as a result of illumination, not only from above the wafer but also from obliquely above it. In  FIG. 15 , an illuminative optical system  10  has basically the same elements as the one shown in  FIG. 2B  and includes a low-angle illumination optical system that includes a laser light source  11  and a high-angle illumination optical system that includes a wide-band light source  12 . Parts assigned the same reference numerals as those shown in  FIG. 2B  and parts located at the same positions of parts shown in  FIG. 2B  but assigned no reference numeral have the same abilities as those described in conjunction with  FIG. 2B . 
     In the configuration shown in  FIG. 15 , laser light emitted from the laser light source  11 , having the diameter thereof enlarged by a beam enlargement optical system  16 , and reflected in the direction of a path L 3  by a branching optical element  218  passes through a mirror  251 , a beam diameter correction optical system  252 , mirrors  253  and  254 , a cylindrical lens  255 , and a mirror  256 . The light is then irradiated to an area  201 - 1  on the surface of a wafer  1  in an illuminating direction  250  at an illuminating angle γ in the form of a slit-shaped beam  201 . A detective optical system composed of an optical filter  660 , an objective lens  620 , a spatial filter  650 , an image formation lens  630 , and a detector  640  is disposed in a direction  260  that intersects the illuminating direction  250  and that meets a Y-axis direction at a horizontal angle Φ (not shown) and meets the surface of the wafer  1  at a detecting angle θ. The slit-shaped beam  201  is irradiated to the wafer in the illuminating direction  250 , whereby light laterally scattered from a foreign matter present on the surface of a thin film coated over the wafer is detected. The light receiving surface of the detector  640  and an area on the surface of the wafer to which the slit-shaped beam  201  is irradiated have a relationship of image formation. The power of the image formation lens  630  is determined so that the light receiving surface of the detector will cover an entire range illuminated with the slit-shaped beam  201 . 
     On the other hand, light scattered upward from a foreign matter existent on the surface of a thin film coated over a wafer illuminated with the slit-shaped beam  201  in the illuminating direction  250  is converged on an objective lens  21 . Moreover, out of light reflected or scattered from the wafer  1  due to wide-band illumination light waves emitted from illumination light emission ends  121   a  and  121   b  of optical fibers  2102  to  2109  shown in  FIG. 6  by which light that falls within a wide wavelength band and is emitted from an illuminative optical system  12  is branched into a plurality of paths, upward scattered light is converged on the objective lens  21 . As described in relation to the first embodiment, the scattered light waves have the wavelengths thereof separated from each other and are then detected by photodetectors  26   a  and  26   b  respectively. 
     A detective system is designed to form an image. This has the merits that stray light other than light reflected or scattered from an object of detection is prevented from giving an adverse effect and that inspection is speeded up because parallel processing can be performed. An automatic focusing control system that is not shown controls a detector so that the light receiving surface  203  of the detector will be disposed in a range illuminated with the slit-shaped beam  201 . Thus, the surface of the wafer is, without fail, located at a certain position in the Z direction during inspection. 
     As shown in  FIG. 15 , the present embodiment includes, in addition to a detective optical system whose elements range from the objective lens  21  to the photodetectors  26   a  and  26   b  and which is disposed perpendicularly to the surface of the wafer  1 , a first oblique detection system whose elements range from a wavelength selection filter  660  to a photodetector  640 , and a second oblique detection system whose elements range from a wavelength selection filter  661  to a photodetector  641 . The detector  640  included in the first oblique detection system and the detector  641  included in the second oblique detection system are, similarly to the detectors  26   a  and  26   b , realized with TDI image sensors respectively. Moreover, a spatial filter  650  or  651  having the same ability as the spatial filter  22  described in conjunction with  FIG. 1  is disposed along the optical axis of the oblique detection system in order to intercept light reflected and diffracted by patterns. Furthermore, a thin film that selectively transmits light which has the same wavelength as laser light emitted from a laser light source  11  is coated over the surfaces of the wavelength selection filters  660  and  661  respectively. Thus, a component of light reflected or scattered from the wafer  1  owing to high-angle illumination achieved by a wide-wavelength band light source  12  is cut and detected. Lenses  620  and  621  are objective lenses, and lenses  630  and  631  are image formation lenses. 
     The position of the second oblique detection system is not limited to the one shown in  FIG. 15  but the second oblique detection system may be opposed to the first oblique detection system. When the second oblique detection system is opposed to the first oblique detection system, if the wafer  1  is illuminated obliquely in the illuminating direction  250 , the results of detections performed by the first and second detection systems respectively are combined in order to obtain a large number of pieces of information on reflected or scattered light waves. 
     As for the illuminating direction, illumination may be performed in a direction  220  or a direction  230 . However, an illuminating means and a detective optical system composed of the image formation lens  630  and detector  640  should preferably be disposed for fear they may be interfere with each other, and directed or angled in order to avoid an adverse effect of light reflected from the substrate such as light diffracted by patterns. Namely, the illuminating means and detective optical system should preferably be located at experimentally determined optimal positions. 
     Image signals detected by the detector  640  included in the first oblique detection system and the detector  641  included in the second oblique detection system respectively are manipulated by signal processing systems  40   c  and  40   d  respectively. The manipulation of the detected image signals is identical to that described in conjunction with  FIG. 13 . An iterative description will be omitted. 
     Third Embodiment 
       FIG. 16  shows an embodiment of a defect inspection system including a microscope. In the present embodiment, a foreign matter detected during inspection can be verified using an observational optical system  60 . 
     A detected foreign matter (including a quasi foreign matter) on the wafer  1  is moved to a position within a field of view offered by a microscope included in the observational optical system  60  by moving stages  31  and  32 . The foreign matter is then observed. 
     The advantage provided by the inclusion of the observational optical system  60  lies in a point that a detected foreign matter can be immediately observed without the necessity of moving a wafer to a reviewer such as a scanning electron microscope (SEM). Since a foreign matter detected by an inspection system can be immediately observed, a cause of occurrence of the foreign matter can be identified quickly. Moreover, an image of a detected foreign matter picked up by a TV camera  64  included in the observational optical system  60  is displayed on a color monitor that is shared by a personal computer. Also included is a facility capable of irradiating laser light to part of a wafer centered on a detected foreign matter, scanning the wafer by moving stages for inspection, marking a scattered light image of the foreign matter and the position of the foreign matter, and displaying the marked image and position on a monitor. Consequently, whether a foreign matter is actually detected can be verified. As an image of part of a wafer scanned by moving the stages, an image of a die adjoining a die in which a foreign matter is detected may be picked up. Comparison and verification can be achieved immediately. 
     As a microscope included in the observational optical system  60 , either a microscope employing a light source that emits visible light (for example, white light) or a microscope employing a light source that emits ultraviolet light may be adopted. For observation of an especially microscopic foreign matter, a high-resolution microscope, for example, the microscope based on ultraviolet light is preferred. The adoption of the microscope based on visible light provides color information on a foreign matter. This is advantageous in that the foreign matter can be readily recognized. 
     Fourth Embodiment 
     Referring to  FIG. 17  to  FIG. 19 , a description will be made of another embodiment that adopts as the low-angle illumination light source  11  an ultraviolet laser such as a KrF laser or an ArF laser and that includes the low-angle illumination optical system  10  included the arrangement shown in  FIG. 2B  or  FIG. 15 . 
     An amount of light scattered from a microscopic particle of 0.1 μm or less in diameter is inversely proportional to the fourth power of the wavelength of illumination light. Therefore, the employment of light having a short wavelength permits higher sensitivity. In order to improve the sensitivity in detecting a defect, an ultraviolet laser emitting light of a shorter wavelength is adopted as an illumination light source. Moreover, for detection of light reflected or scattered from a microscopic foreign matter or defect, an amount of illumination light should be large. 
     When a pulsed oscillation laser is adopted as the ultraviolet laser, a peak value (maximum power) of power provided by the pulsed oscillation laser is much larger than a required mean power. For example, assuming that the mean power of a laser is 2 W, the emission frequency of light emitted therefrom is 100 MHz, the pulse spacing thereof is 10 ns, and the pulse width thereof is 10 ps, the peak value (maximum power) is so large as 2 kW that it may damage a specimen. Consequently, the peak value (maximum power) should preferably be decreased with the mean power held intact. 
     As for a method of decreasing the peak value with the mean power sustained, as shown in  FIG. 17 , a laser beam L 0  emitted from the light source  11  is enlarged by a beam enlargement optical system  16 , and routed to a pulse branching optical system  17  so that the laser beam will be branched into a plurality of paths whose path lengths are different from one another. Thereafter, the light paths are integrated into one. Thus, one pulse of laser light emitted from the light source is divided into a plurality of pulses having a smaller peak value. The plurality of pulsed laser light waves is routed to a branching optical element  218  so that the light waves will be introduced to any of the directions of paths L 1 , L 2 , or L 3  shown in  FIG. 2B  or  FIG. 15 . Thus, the light waves are recomposed into a slit-shaped beam which is then irradiated to a slit-shaped area  201 - 1  on the wafer  1 . 
     Since a pulsed laser beam is split into a plurality of components and then irradiated, assuming that the moving speed of an X stage  31 - 1  on which a substrate W to be inspected is placed is 20 cm per sec and the size of a field of view offered by one pixel location in a photodetector  26   a  or  26   b  is 1 μm, when an ultraviolet pulsed laser beam whose frequency is 100 MHz is split into a plurality of components under the aforesaid conditions, several hundreds or more of pulses of laser light are repeatedly irradiated to an area to be detected by one pixel location in the detector  26   a  or  26   b . Consequently, speckle noises derived from a laser beam can be temporally averaged for imaging. This results in an image having noises minimized. 
       FIG. 18A  shows an example of a pulsed light split optical system  17 . In this example, the pulsed light split optical system  17  includes quarter-wave plates  1711   a  and  1711   b , polarization beam splitters (PBS)  1712   a  and  1712   b , and mirrors  1713   a  and  1713   b . The quarter-wave plate  1711   a  transforms an incident laser beam, which is enlarged by the beam enlargement optical system  16  and is linearly polarized (p-polarized light in this example), into elliptically polarized light. The polarization beam splitter  1712   a  separates the elliptically polarized light into p-polarized light and s-polarized light. One of the polarized light waves, that is, the p-polarized light passes through the polarization beam splitter  1712   a  and polarization beam splitter  1712   b . The other s-polarized light is reflected from the polarization beam splitter  1712   a , mirrors  1713   a  and  1713   b , and polarization beam splitter  1712   b , and returned to the same optical axis as the p-polarized light having passed through the polarization beam splitters  1712   a  and  1712   b  is. At this time, assuming that the spacing between the polarization beam splitter  1712   a  and mirror  1713   a  or between the polarization beam splitter  1712   b  and mirror  1713   b  is L/2 m, the path of the s-polarized light and the path of the p-polarized light have an optical path difference of L m. Assuming that the light velocity is c m/s, the s-polarized light and p-polarized light have a temporal difference expressed by formula 2 below. 
         t  (s)= L  (m)/ c  (m/s)  (2)
 
     A beam containing two pulses that have a time interval T between them as shown in  FIG. 18B  and being emitted from the laser light source  11  is temporally split. Consequently, as shown in  FIG. 18C , one pulse is split into two pulses having a time interval t between them in order to halve the peak value of the pulse. 
     For example, assume that laser light having a pulse spacing of 10 nm (10 −8  sec) and a pulse width of 10 ps (10 −11  sec) is employed and the spacing between the polarization beam splitter  1712   a  and mirror  1713   a  or between the polarization beam splitter  1712   b  and mirror  1713   b  is set to 15 cm (0.15 m). In this case, the temporal difference between the s-polarized light and p-polarized light is 1 ns (10 −9  sec). Consequently, a pulsed laser beam whose peak value is halved is irradiated to the surface of a wafer twice at intervals of 1 ns within 10 ns. 
     The angle of rotation of the quarter-wave plate  1711   a  is adjusted so that the ratio of s-polarized light to p-polarized light in a beam incident on the polarization beam splitter  1712   a  will be 1:1 (circularly polarized light). Due to a loss in reflectance or transmittance caused by employed optical elements (polarization beam splitters  1712   a  and  1712   b  and mirrors  1713   a  and  1713   b ), the peak values of s-polarized pulsed light and p-polarized pulsed light separated from a beam emitted from the polarization beam splitter  1712   b  become different from each other. In order to decrease a maximum value to be assumed by the peaks of respective pulsed light waves, the peak values of the respective pulsed light waves must be equal to each other. 
     According to the configuration of the pulse split optical system  17  shown in  FIG. 18A , p-polarized light is affected by the transmittance (Tp) of p-polarized light offered by the polarization beam splitters  1712   a  and  1712   b . In contrast, s-polarized light is affected by both the reflectance (Rs) of s-polarized light offered by the polarization beam splitters  1712   a  and  1712   b  and the reflectance (Rm) of s-polarized light offered by the mirrors  1713   a  and  1713   b . Assuming that Ls denotes a loss of s-polarized light and Lp denotes a loss of p-polarized light, a loss ratio (Pl) is expressed by formula 3 below. 
         Pl=Ls/Lp=Rm   2   ×Rs   2   /Tp   2   (3)
 
     Consequently, when the angle of rotation of the quarter-wave plate  1711   a  is adjusted so that the ellipticity of polarized light incident on the polarization beam splitter  1712   a  will be equal to the loss ratio, the peak values of s-polarized pulsed light and p-polarized pulsed light into which light emitted from the polarization beam splitter  1712   b  is separated become nearly equal to each other. The p-polarized pulsed light and s-polarized pulsed light that are separated to have the nearly equal peak value are transmitted by the quarter-wave plate  1711   b  to become circularly polarized light waves. 
     The method of halving pulsed light using the pulse split optical system  17  has been described. Referring to  FIG. 19A  and  FIG. 19B , a method of quartering pulsed light will be described in relation to a variant of the pulse split optical system  17  that splits pulsed light into a larger number of components. The configuration of a pulse split optical system  17 ′ shown in  FIG. 19A  includes two stages of the pulse split optical system  17  shown in  FIG. 18A . The spacing between a polarization beam splitter  1732   c  and a mirror  1733   c  or between a polarization beam splitter  1732   d  and a mirror  1733   d  included in the second stage is set to a double of the spacing between a polarization beam splitter  1732   a  and a mirror  1733   a  or between a polarization beam splitter  1732   b  and a mirror  1733   b  included in the first stage. Light emitted from the polarization beam splitter  1732   b  in the first stage includes p-polarized pulsed light and s-polarized pulsed light that lags behind the p-polarized pulsed light. The optical pulse train is recomposed into circularly polarized light by a quarter-wave plate  1731   b . P-polarized light whose intensity is a half of the intensity of the optical pulse train transmitted by the quarter-wave plate  1731   b  is transmitted by the polarization beam splitters  1732   c  and  1732   d . S-polarized light whose intensity is a half of the intensity of the pulse train is reflected from the polarization beam splitter  1732   c  and mirrors  1733   c  and  1733   d , reflected from the polarization beam splitter  1732   d , and returned to the same optical axis as the p-polarized light is. Consequently, pulsed light is quartered, and the peak values of the quarters are a quarter of the peak value of the pulsed laser beam emitted from the light source  11 . Strictly speaking, since optical elements causes loses in reflectance or transmittance as mentioned above, the peak value is smaller than the quarter. 
     In the configuration shown in  FIG. 19A , p-polarized pulsed laser light having passed through the polarization beam splitters  1732   c  and  1732   d  and s-polarized pulsed laser light reflected from the mirror  1733   d  and polarization beam splitter  1732   d  propagate along the same optical axis, are recomposed into circularly polarized light by the quarter-wave plate  1731   c , and then falls on the polarization beam splitter  1734  (equivalent to the branching optical element  218  shown  FIG. 2B  or  FIG. 15 ). Consequently, the circularly polarized light is separated into p-polarized light and s-polarized light. One of the polarized light waves, that is, the p-polarized laser light is propagated along a path L 2 ′ equivalent to the path L 2  shown in  FIG. 2B  or  FIG. 15 , and then reshaped by a cylindrical lens  1735  (equivalent to the cylindrical lens  244  in the path L 2  shown in  FIG. 2B  or  FIG. 15 ). The resultant light illuminates a linear area  1793  on the wafer  1  (equivalent to the linear area  201 - 1  on the wafer  1  shown in  FIG. 2B  or  FIG. 15 ). 
     On the other hand, the s-polarized light reflected from the polarization beam splitter  1734  and thus angled 90° is propagated along a path L 3 ′ equivalent to the path L 3  shown in  FIG. 2B  or  FIG. 15 , reflected from mirrors  1736  and  1737  to have the path changed to another, and then reshaped by a cylindrical lens  1738  (equivalent to the cylindrical lens  255  in the path L 3  shown in  FIG. 2B  or  FIG. 15 ). The resultant light illuminates, unlike p-polarized laser light that illuminates the wafer in the direction of the path L 2 ′, the linear area  1793  on the wafer  1  perpendicularly to the wafer  1 . 
     The path L 2 ′ and path L 3 ′ are designed so that the optical lengths thereof will be different from each other. As shown in  FIG. 19B , p-polarized laser light and s-polarized light that are irradiated to the linear area  1793  on the wafer  1  are irradiated at different timings that have a temporal difference t 0  proportional to an optical path difference. Consequently, interference between the p-polarized laser light and s-polarized laser light that are irradiated to the linear area  1793  can be prevented. 
     Moreover, the photodetector  26   a  that detects light reflected or scattered under illumination with the laser light source  11  detects light reflected or scattered under illumination performed in a direction deviated by 90° within a period of time during which the photodetector detects one pixel. Consequently, a variance in detective sensitivity attributable to a difference in an illuminating direction can be minimized. Eventually, a more microscopic foreign matter or defect can be stably detected. At this time, a direction of detection is the direction of an arrow  1740 . Moreover, as for the photodetector, either or both of the detectors  640  and  641  described in conjunction with  FIG. 15  may be employed. 
     According to the present embodiment, an ultraviolet pulsed laser beam can be irradiated to a wafer with the peak value thereof decreased. A quite microscopic defect having a diameter of about 0.1 μm or less can be detected without any damage to the wafer. 
     According to the present invention, an object of inspection is illuminated with single-wavelength light and wide-wavelength band light, which are different from each other in a wavelength band, in efforts to stably detect a defect on the surface of the object of inspection. A spatial filter having the ability to intercept light diffracted by repetitive patterns and pass light diffracted by non-repetitive patterns is disposed at a position in a detective optical system at which a Fourier transform is observed. A means is included for designating conditions for the spatial filter on the basis of an image of light diffracted by actual patterns which is viewed at the position of a Fourier transform, or an image of light diffracted from patterns which is supposed to be viewed on the image plane of a Fourier transform and is produced based on design data representing patterns formed on the surface of the object of inspection. Light scattered from the surface of a specimen and transmitted by the spatial filter is separated into light components of different wavelengths. An image is detected by a detector and compared with an image of an adjoining die within the same specimen in order to detect a microscopic defect. Thus, light diffracted by circuit patterns on a substrate such as LSI patterns is reduced so that a microscopic foreign matter or defect, a foreign matter or defect causing a short circuit of wiring, or a thin-film-like foreign matter can be detected quickly and highly precisely. 
     Moreover, according to the present invention, one pulse of laser light emitted from a pulsed laser light source designed for low-angle illumination is split into a plurality of pulses in order to decrease the peak value. The resultant laser light is irradiated to a specimen. Consequently, a high-luminance pulsed laser may be used to detect a more microscopic foreign matter. Nevertheless, the more microscopic foreign matter can be stably detected without any damage to the specimen. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.