Abstract: Practical third-order frequency-resolved optical grating (FROG) techniques for characterization of ultrashort optical pulses are disclosed. The techniques are particularly suited to the measurement of single and/or weak optical pulses having pulse durations in the picosecond and subpicosecond regime. The relative quantum inefficiency of third-order nonlinear optical effects is compensated for through i) use of phase-matched transient grating beam geometry to maximize interaction length, and ii) use of interface-enhanced third-harmonic generation.

Abstract: An apparatus and method for characterizing light beam collimation and alignment. A liquid crystal device receiving a light beam from a collimation lens has a lenslet array generated thereon. A corresponding array of point spread functions are detected by a charge coupled device camera, from which light beam collimation and alignment are characterized. The data may be further used to adjust the collimation lens to correct misalignment.

Abstract: A lensless joint transform correlator optically determines the relative position of two pinholes placed in parallel arms of a Mach-Zehnder interferometer. The Fraunhaufer diffraction patterns combine to form a joint power spectrum signal in the joint transform plane. Because of the interference of the two pinhole signals, an encoded lens comprising a Fresnel zone is formed in the joint Fraunhaufer diffraction pattern which is illuminated with a plane wave, and the output signal is taken in the focal plane of the Fresnel zone and magnifies the input pinhole displacement by a factor of ten, enhancing its use in precision positioning servo systems for mask alignment.

Abstract: Optical systems are disclosed which are capable of generating and rapidly changing time delays of electrical signals for true time delay beam formation and beam steering and for signal processing applications. The systems utilize an interferometer configuration. A first optical modulator in a first leg of the interferometer is used to modulate coherent light with the signal to be delayed. In a second leg of the interferometer, a second optical modulator provides beam steering to a prism stack, which produces a set of plane reference waves having a range of orientations required to generate a desired range of time delays. Preferably the optical modulators are acousto optic Bragg cells. Alternatively, a stack of lens pairs or diffractive optical elements or a holographic optic element may be used in place of the prism stack. The modulated optical signal from the first leg interferes on an array of photodiodes with the reference waves from the second leg.

Abstract: In an optical detector, a light source irradiates coherent light onto an objective. A Fourier transform lens receives the light diffracted and scattered at the objective and Fourier transforms the light to generate a Fourier image, the Fourier image having a high intensity spectral component corresponding to the periodic pattern on the objective and a low intensity spectral component corresponding to the abnormal portion. In an optically-addressed spatial light modulator, each of the optically-addressing part and the light modulating part receives the Fourier image at the corresponding portions.

Abstract: In an optical detector, a light source irradiates coherent light onto an objective. A Fourier transform lens receives the light diffracted and scattered at the objective and Fourier transforms the light to generate a Fourier image, the Fourier image having a high intensity spectral component corresponding to the periodic pattern on the objective and a low intensity spectral component corresponding to the abnormal portion. In an optically-addressed spatial light modulator, each of the optically-addressing part and the light modulating part receives the Fourier image at the corresponding portions.

Abstract: An optical image processor which may be used for optical image correlation comprises a liquid crystal spatial light modulator for displaying an input image as a two dimensional array of pixels. An array of photodetectors provides the output. Between the SLM and the photodetectors, there are provided a spatial light modulator and microoptic array of pin holes or lenses. The SLM has a respective picture element for each of the elements of the array and displays a filter or template image for correlation with the images displayed on the input SLM. Each photodetector of the array of output photodetectors views each of the pixels of the SLM via respective pin holes or lenses and pixels of the SLM and array. Thus, each photodetector receives light from the input through an array of pin holes or microlenses which, when selectively shuttered, act as a filter.

Abstract: In a system for measuring variations in thickness of an optical etalon, a light source and a diffraction grating are mounted on a base structure with an axle. A lever arm is affixed to the axle, and a micrometer is held in contact with the lever arm. The grating directs a into an optical path a wavelength of radiation dependent on orientation of the grating. The etalon is supported in the optical path to effect a fringe pattern representing variations in thickness of in the etalon. The orientation is varied with the micrometer so as to vary the wavelength to the etalon and thereby positioning of the fringe pattern across the etalon which is viewed through a microscope. The micrometer measures the variation of orientation and thereby variation in thickness across the interference element.

Abstract: The object of the invention is to provide a method which makes it possible to determine the correlation between two optical signals, which makes it possible to correlate two optical signals, without using a crystal element which is associated with a plurality of drawbacks, because the components are quite expensive. It has been found possible to replace a frequency doubling crystal by an active, optical wave guide, e.g. in the form of an active, optical fiber, said wave guide transmitting fluorescence which is intensity-related to the arrived signal. When this fluorescence is detected in spectral bands, e.g. in the blue region of the visible light, an image of the strength of the arriving signal may be formed. Current distance delaying of the two correlated signals makes it possible to produce an image of the correlation between the signals.

Abstract: Method and apparatus for forming an image of an object by optical projection with a focusing system comprising the steps of collimating a set of at least two mutually incoherent light beams from the light source to fall on the object from directions that makes the non-diffracted components intersect the aperture stop at a set of points that are distributed over the surface of the stop, attenuating in a spatial filter the zero diffraction order of each beam relative to diffraction orders diffracted in directions towards the centre of the aperture stop, and adding the images formed by the light beams.

Abstract: An electroptic metrology system uses Moire interferometry to generate contour information on the surface of an object. Interfering coherent beams generate a n image on the object's surface that includes a stripe pattern. The surface is viewed with a high resolution camera and digitized using a frame grabber and computer. A reference pattern is generated in software and is mixed with the digitized object surface signals. The mixed signals are filtered to remove signals at the carrier frequency and signals containing the beat frequency indicative of the surface contours are output.

Abstract: An apparatus and method for inspecting and guiding a web utilizes a curved outer convex cloth guiding surface (A) on arcuate sections such as extrusions provided respectively at the top and bottom of the viewing area for guiding cloth and for forming a housing for the light box wherein an inner housing surface (B) is provided opposite the cloth guiding surface (A) on each extrusion. Depending receiving members (C) are provided at each respective end of the arcuate extrusions for serving as a receptacle for guiding and positioning a plexiglass viewing member and back sheet metal wall for forming a light box. A force applying member (D) is provided for controlling sag or bow in an extrusion. Apparatus and method is illustrated including diverging grooves for spreading the web as it passes over the upper section, and circuitry is provided conserving energy by limiting the time the circuitry is energized.

Abstract: An optical correlator uses ferroelectric liquid crystal spatial light modulators (FLC-SLM's) in both the reference and filter planes. The SLM's include an electrically addressable memory to store images in the form a two-dimensional array of reflective pixels beneath the FLC layer. The SLM's selectively rotate the polarization of the light reflected by each pixel in accordance with the stored image. In particular, a laser produces a polarized beam that is directed through a first polarizing beamsplitter and onto the reference SLM. This beamsplitter blocks unmodulated light reflected by the reference SLM and transmits modulated light through a set of Fourier tranform lenses. The resulting beam is directed through a second polarizing beam splitter onto a filter SLM that has been programmed with the complex conjugate of the Fourier transform of a desired target image.

Abstract: Optical systems are disclosed which are capable of generating and rapidly changing time delays of electrical signals for true time delay beam formation and beam steering and for signal processing applications. The systems utilize an interferometer configuration. A first optical modulator in a first leg of the interferometer is used to modulate coherent light with the signal to be delayed. In a second leg of the interferometer, a second optical modulator provides beam steering to a prism stack, which produces a set of plane reference waves having a range of orientations required to generate a desired range of time delays. Preferably the optical modulators are acousto optic Bragg cells. Alternatively, a stack of lens pairs or diffractive optical elements or a holographic optic element may be used in place of the prism stack. The modulated optical signal from the first leg interferes on an array of photodiodes with the reference waves from the second leg.

Abstract: A photoelectric conversion apparatus includes a pair of opposite electrodes, and an amorphous silicon film. The amorphous silicon film is sandwiched between the pair of opposite electrodes and serves to perform photoelectric conversion of light including light from a target object. At least one of the pair of opposite electrodes is divided into a set of pattern electrodes respectively connected to the amorphous silicon film in electrically positive and negative directions so as to form a spatial filter electrode. The respective pattern electrodes respectively connected in the positive and negative directions are irregularly arranged on the amorphous silicon film.

Abstract: An inspection apparatus for a light diffracting surface employs a planar array of individually addressable light valves for use as a spatial filter in an imaged Fourier plane of a diffraction pattern, with valves having a stripe geometry corresponding to positions of members of the diffraction pattern, blocking light from those members. The remaining valve stripes, i.e. those not blocking light from diffraction order members, are open for transmission of light. Light directed onto the surface, such as a semiconductor wafer, forms elongated curved diffraction orders from repetitive patterns of circuit features. The curved diffraction orders are transformed to linear orders by a Fourier transform lens. The linear diffraction orders from repetitive patterns of circuit features are blocked, while light from non-repetitive features, such as dirt particles or defects is allowed to pass through the light valves to a detector.

Abstract: An apparatus used to inspect patterned wafers and other substrates with periodic features for the presence of particles, defects and other aperiodic features in which a spatial filter placed in the Fourier plane is used in combination with either broadband illumination, angularly diverse illumination or both. In contrast to prior devices that direct light from a single monochromatic source through a pinhole aperture stop, embodiments are describes that illuminate a patterned substrate using (1) a single monochromatic source with a slit-shaped aperture stop for angularly diverse illumination, (2) a single broadband source with a pinhole aperture stop for broadband illumination, (3) a single broadband source with a slit-shaped aperture stop for both broadband and angularly diverse illumination, or (4) multiple sources with an aperture stop for each source for at least angularly diverse illumination.

Abstract: An object information processing apparatus includes a focussing unit for focussing light information from an object point; a light transmission unit for transmitting the light information from the object point to the focussing unit via first and second optical passages; and mask units provided respectively in the first and second optical passages in the light transmission unit and formed respectively with predetermined patternings which are in negative and positive relation to each other. By virture of the mask units, the light information from the object point is blocked into a plurality of distance informations from which distance patterns are obtained. The apparatus can recognize an object in three dimensions using the distance patterns.

Abstract: In an imaging system (10) for detecting defects in a specimen (14) having a repetitive pattern (16), a spatial filter (50) receives a spatial frequency spectrum produced by a Fourier transform lens (34) and blocks preselected spatial frequency components thereof. The spatial filter includes an array of substantially parallel opaque stripes (70a-70c) that are positioned on a substantially transparent substrate (72). In one embodiment, the stripes are spaced apart by equal distances (78) and are of increasing widths (76a-76c) that correspond to the orders of diffraction of the Fourier transform pattern (45) produced by the Fourier transform lens. The spatial filter can be used to filter light spots forming a Fourier transform pattern for specimens having repetitive pattern sizes included within a specified range of sizes.

Abstract: The Fourier transform optical processing system of the present invention includes generating an electrical signal in response to a first image and modulating a beam of coherent light with means that is responsive to the electrical signal. A Fourier transform image of the modulated coherent light beam is formed, an then detected. The detector provides a second electrical signal representative of the Fourier transformed image. Other aspects of the invention include comparing the second electrical signal to a reference electrical signal for producing an output signal representing any differences; optically preprocessing the image; detecting the Fourier transform image and generating signals representative of its Fourier transform characteristics; and modulating a coherent light beam using a liquid crystal or other spatial light modulating device.

Abstract: A transform digital optical processing system generates a transform signal of an image. Fourier or other well-known transforms may be employed. The transform signal may be generated in one of two ways: optically or electronically. In optical generation a two dimensional object is generated by modulating a beam of coherent light with an image of the object. A transform image of the modulated coherent light beam is formed, using an optical transform element. The optical transform is then stored in a two dimensional buffer. The transform signal may also be generated electronically by storing a digital video image of an object and generating a Fourier or other transform of the digital video image using vector processing chips or other commercially available digital transform generating computers. This digitally generated information may be analyzed and classified through a neural network type processor.

Abstract: Automatic focusing of an interference microscope is accomplished by directly sensing an interference pattern produced by a white light source with an auxiliary point detector. A beamsplitter intercepts part of the interference beam and directs it to the point detector. A narrow band filter filters light passing through the beam splitter on its way to a main detector array. An objective of the interference microscope is rapidly moved to an initial position between a sample surface and a fringe window by operating a position sensor to sense when the objective is a predetermined safe distance from the sample surface and turning off a motor moving the objective. The objective then moves rapidly from the initial position until the presence of fringes is detected by the point detector. Momentum of the microscope causes the objective to overshoot beyond a fringe window.

Abstract: A method of using the classic principles of moire patterns and holography to provide defect analysis of variable sensitivity on structures of various sizes, including large aircraft. By taking several holograms (usually a series of four) of the structure which has been illuminated with dual (or multiple) beams of coherent illumination (e.g. laser), each hologram will record an image of the test object which includes an interferometric fringe pattern on the surface. The spacing and orientation of the fringe pattern can be varied by the arrangement and nature of the dual illumination beams.

Abstract: A method and apparatus for measuring a three-dimensional curved surface shape based on a new measuring principle that a surface of an object to be measured is coded with information relating to a slit light which is used as a medium and moved or rotated at a constant speed. The manner in which a linear reflected pattern of the slit light is moved over the surface of the object to be measured is picked up by a television camera to form a composite image in which a value of each of picture elements in the image is represented by information relating to the slit light, e.g., a position, time or projection angle of the slit light at an instant that the slit light passes through one of positions on the object surface corresponding to that picture element.

Abstract: A high-speed optical fingerprint correlator. The invention uses the unknown ingerprint to produce a matched filter for autocorrelation and detecting and displaying the resultant indicia of correlation, the correlation peaks produced. The matched filter is produced by performing digital Fourier transform on the optically scanned digitized image input of the unknown print. The matched filter is then multiplied with the optically performed Fourier transforms on the reconstructed known fingerprint images provided in digital format from visual or digital storage.

Abstract: In a mask for transmitting a light subjected to a projection on a predetermined position and for detecting the light, transmittances of the mask are distributed in correspondence with a statistical distribution of the predetermined positions of the light. Information represented by optical signals can be detected with a high probability.

Abstract: Automatic focusing of an interference microscope is accomplished by directly sensing an interference pattern produced by a white light source with an auxiliary point detector. A beamsplitter intercepts part of the interference beam and directs it to the point detector. A narrow band filter filters light passing through the beam splitter on its way to a main detector array. A memory lock position of the microscope objective is manually selected and stored. Initially, the objective moves rapidly from the memory lock position until the presence of fringes is detected by the point detector. Momentum of the microscope causes the objective to overshoot beyond a fringe window. The microscope objective then is moved more slowly through the interference window until fringes again are detected; the lower speed results in substantially reduced overshoot. Intensity measurements from the point detector are sensed and stored as the objective moves through the width of the fringe window.

Abstract: An optical radar system for coherent ranging and velocimetry. Modulation of a laser diode by light backscattered from a target is used to determine distance and velocity of a target. An array of laser diodes are used to determine the velocity and topography of a target. Three laser diodes are used to determine speed and orientation of a rotating disk.

Abstract: A method and apparatus adapted to measure a three-dimensional curved surface shape quantitatively with high speed. A pattern of interference fringes on a surface of an object to be measured is picked up so that while moving a reference plane (or the object to be measured) in the direction of an optical axis, the resulting video signal is processed to form a composite image in which a value of each of picture elements is represented by a position of the reference plane (or a position of the object to be measured) at an instant that one of positions on the object surface corresponding to that picture element in the image attains a maximum brightness. Then, a three-dimensional curved surface shape of the object to be measured is measured on the basis of the composite image.

Abstract: A dynamically matched optical filter is formed within a multiple telescope optical imaging system. Entrance and exit pupil geometries, the phases and tilts of multiple imaging paths and system alignments are sensed and controlled to allow wide field of view imaging and the enhancement or suppression of certain spatial frequencies of interest.

Abstract: Electro-optical apparatus measures the average relative phase of an incident wave fringe pattern. The subject fringe, e.g., an interferometric pattern, passes through three sections of an optical mask, one characterized by fixed transmissivity and the other two by quadrature-displaced spatial fringe patterns. The light passing through each section is separately collected and detected to average the respective incident wave/mask section interactions. The phase of the incident fringe pattern relative to the mask is then determined by arithmetically processing the detected signals.In accordance with one aspect of the present invention, the subject fringe pattern is time modulated and the quadrature-shifted mask signals A-C coupled to obviate the requirement for the third, fixed transmissivity mask section.

Abstract: An electro-optical device (10) is used to alter characteristics of a light beam passing through it. In a particularly preferred embodiment, the device (10) is used as a phase stop having a dynamically variable aperture whose size is defined by the selected application of an electrical potential across one or more concentric, transparent ring-like electrodes (13, 14, 16 and 18). The electro-optical device (10) finds particular utility in a common-path interferometer (40) where fringe visibility is optimized by using suitable servo-electronics (56) to select an appropriate aperture size for the phase stop (10).

Abstract: An inspection system (10, 100) employs a Fourier transform lens (34, 120) and an inverse Fourier transform lens (54, 142) positioned along an optic axis (48, 144) to produce from an illuminated area of a patterned specimen wafer (12) a spatial frequency spectrum whose frequency components can be selectively filtered to produce an image pattern of defects in the illuminated area of the wafer. Depending on the optical component configuration of the inspection system, the filtering can be accomplished by a spatial filter of either the transmissive (50) or reflective (102) type. The lenses collect light diffracted by a wafer die (14) aligned with the optic axis and light diffracted by other wafer dies proximately located to such die. The inspection system is useful for inspecting only dies having many redundant circuit patterns.

Abstract: The measurement range of Moire contouring techniques is extended beyond the Nyquist frequency limit by constraining the reconstruction of a surface contour according to a priori knowledge about the surface. In one example the a priori knowledge is that the surface is smooth, being described by a function having continuous derivatives, and the location of a region where the surface changes by less than C/2 per sample. In another example, the a priori knowledge is the location and height of a step discontinuity to within C/2, where C is the contour interval.

Abstract: A method for accurate phase determination in holographic interferometry using a one- to two-dimensional Fourier transform is described. The method calculates the interference phase pointwise, even between fringe extrema, and thus has advantages over conventional fringe-finding and -tracking methods. Only one interference pattern may be used, although the use of two patterns reconstructed with a mutual phase shift permits an easier phase unwrapping and determination of nonmonotonic fringe-order variations. Additionally, the method offers a means for filtering out disturbances such as speckle noise and background variations.

Abstract: An improved interferometer is provided which is of simplified construction and gives enhanced, accurate interference analysis through provision of an alterable masking device located at the interference plane which facilitates rapid computer analysis of an interference pattern. The interferometer of the invention preferably includes a source of electromagnetic radiation, a beam splitter, a lens for convergence and interference of the beams, a masking device located at the region of beam convergence, and detection apparatus for receiving and analyzing radiation from the mask. The mask is preferably stationary and electrically alterable and advantageously includes a base along with separate, plural masking structures mounted on one base face with electrical connections coupled to the separate masking structures.

Abstract: A laser radiation detection system is disclosed comprising phase retarding apparatus for retarding a portion of the laser radiation incident on the system, optical focusing apparatus, detector apparatus and an electrical circuit. The optical focusing apparatus focuses the incident radiation on the detector apparatus. The detector apparatus is located in the focal plane and detects the presence of laser radiation due to the uneven distribution of light caused by the interference of the retarded portion of the incident radiation with the portion that remains in phase. The detector apparatus supplies output signals in response to the incident radiation received by each detector to the electrical circuit which then compares the various outputs from the detector apparatus in pairs and outputs a signal indicating the presence of laser radiation when the sum of the differences of the detector outputs is greater than a predetermined threshold value.

Abstract: An optical pressure sensor 10 for measuring the pressure within a container 12 such as a combustion chamber of a rocket engine, wherein pressure sensor 10 comprises a pressure deflectable diaphragm 14 in communication with container 12 and which includes an optical grating 22 on one of its surfaces, means 18 for directing a laser 20 to strike optical grating 22 from a fixed direction to form a central diffracted beam 24 and a first order diffracted beam 26, a photo-position detector 60 for generating signals responsive to the changes in position of the central and first order diffracted beams 24 and 26 resulting from the effects of pressure and temperature upon the diaphragm 14, and means 30 for synthesizing a temperature-adjusted indication of the pressure in the subject container from said responsive signals.

Abstract: A microscope, not employing conventional optical elements, is disclosed wherein a sample under examination is subjected to a moving pattern of illumination having alternate light and dark interference fringes, and having a spatial frequency which varies during the motion of the fringes over the sample in a first direction, in order to obtain the sine components of the optical image of the sample. The light reflected off of the sample is gathered by a wide angle light detector to produce electrical signals which in turn result in the recording of a set of signals proportional to the intensity of the light reflected from the sample during a plurality of signal sampling intervals. Another set of signals is recorded after the direction of relative motion between the pattern and the sample is changed by about 1.degree., for example, and so on until electrical data is recorded for a considerable number of "fourier slices" across the sample object.

Abstract: The physical separation between a rear illuminated opaque pattern on a translucent substrate and an optical pattern sensing device viewing the shadow image of the opaque pattern is increased without any corresponding loss of resolution (and/or resolution may be increased without any corresponding reduction in the physical separation between the optical pattern sensing device and the opaque pattern) by positioning a narrow spectral band pass interference filter between the opaque pattern and the optical pattern sensing device, and using as the rear illumination, light having a spectrally narrow band which substantially matches the pass band of the interference filter.

Abstract: A lift control system includes a spatial filter for producing a signal having various features, a discriminator for discriminating one or more features of the signal, and for producing a control signal when at least one of the features discriminated falls in a predetermined range, and active devices connected to the discriminator. The spatial filter is defined by a semi-spherical bowl having a plurality of openings or by a poly-face prism which is made from a plano-convex lens with its curved face so polished or cut as to have a plurality of flat faces.

Abstract: An encoding disk including a multiplicity of linear gratings placed accurately on the disk to diffract a light beam to a fixed point in space accurately does so independently of the centering of the disk on its axis of rotation.

Abstract: Interference patterns are generated in response to temporally coherent light incident upon an interferometer which directs the interference patterns upon an array of photosensitive elements. The photosensitive elements are connected to generate signals representative of the optical energy respectively incident on the elements. The output signals of the photosensitive elements are averaged by a passive resistor network coupled to the photosensitive elements and the signals generated by the individual elements are compared to the average signal by individual comparator circuits to detect the presence and position of an interference pattern directed upon the array.

Abstract: Image comparison is accomplished by forming a composite image composed of a reference image and a test image. A two-dimensional image spectrum is generated from the composite image and then is whitened by setting the magnitude of every point of the two-dimensional image spectrum to a uniform level. A phase-only image of the composite image is then constructed, and values of the phase-only image exceeding a predetermined threshold are detected as an indication of the location of a difference between the reference and test images.

Abstract: The temporal shape of optical pulses is measured over a wide dynamic range, for example, 10 orders of magnitude, by passing an optical signal corresponding to the autocorrelation function of the optical pulses through a variable attenuation filter, the position of which is a function of the attenuation. By plotting the attenuation of the filter in terms of the position thereof, against the duration of the temporal overlap of the pulses in a mixing crystal which produces the optical signal corresponding to the autocorrelation function, the temporal shape of the pulses is displayed.

Abstract: A technique for automatically centering a lightguide fiber (7) in a transparent plastic coating (6) having a refractive index lower than that of the fiber. The fiber (7) is passed through an applicator (14) having the coating material (6) therein to coat the fiber. Orthogonal laser beams (50 or 52) are directed at the coated fiber (5) resulting in first and second forward scattered light patterns (51 or 53) impinging on first and second screens (36 and 37). The patterns are monitored with a pair of CCTV cameras (38 and 39) and the video output signals therefrom are processed to determine the period (P.sub.1 and P.sub.2) of outboard interference fringes (61 and 62) of each pattern. The period of the interference fringes are compared to determine the difference therebetween which is proportional to the eccentricity of the fiber (7) within the coating. The position of the applicator (14) is then adjusted to center the lightguide fiber (7) in the coating (6).

Abstract: A low pass filter for an auto focus camera system to deflect light from the main lens by a controlled angular amount and thus suppress high frequency components in the signal.

Abstract: An apparatus for altering the spatial frequency content of recorded patterns by selectively filtering the patterns after they are transformed into the Fourier domain. The filter characteristics are spatially and temporally variable while the real space image of the altered pattern is being observed. A laser beam is projected through a transparency containing the pattern. The patterned beam, with diffraction interference effects, is transformed to the Fourier domain at the surface of a liquid crystal light valve responsive to a spatially and temporally varying light projector. The projector activated regions reflect areas of the Fourier domain beam, while other areas are absorbed or incoherently reflected. A beam splitter redirects a portion of the reflected beam to a continuously observable image plane after transformation back into real space.

Abstract: A defect detection apparatus is disclosed for optically inspecting a spiral groove of a video disc record by directing a coherent light beam at the grooved surface of the disc. The incident beam illuminates the grooved surface with a light spot that spans a plurality of groove convolutions. The structure of the illuminated region on the grooved surface serves as a diffraction grating for diffracting the incident beam into discrete diffraction orders. Relative motion is established between the disc surface and the incident beam in a manner causing the illuminating spot to rapidly scan the grooved surface in a coarse spiral pattern. A lens system is positioned to collect the light from a single beam of a particular group of non-zero diffraction order beams and to focus this non-zero order beam onto a photodetector. When a defect in the groove pattern exists in the illuminated region the measured power of the non-zero order beam will fall below its normal level.

Abstract: An inspection system (10, 100) employs a Fourier transform lens (34, 120) and an inverse Fourier transform lens (54, 142) positioned along an optic axis (48, 144) to produce from an illuminated area of a patterned specimen wafer (12) a spatial frequency spectrum whose frequency components can be selectively filtered to produce an image pattern of defects in the illuminated area of the wafer. Depending on the optical component configuration of the inspection system, the filtering can be accomplished by a spatial filter of either the transmissive (50) or reflective (102) type. The lenses collect light diffracted by a wafer die (14) aligned with the optic axis and light diffracted by other wafer dies proximately located to such die. The inspection system is useful for inspecting only dies having many redundant circuit patterns.