Abstract: An inspection system (2) employs a beam of monochromatic light (12) that travels through a Fourier transform lens (16) before striking a specimen wafer (4) at an angle (.THETA.) with respect to the normal (26) of the specimen wafer (4) to produce diffracted light (28b ) and 28c) that has a broad spatial frequency spectrum which can be selectively filtered to produce a dark field image pattern of the various sized defects in an inspection area (22) of the wafer. The nearly collimated beam of monochromatic light strikes the wafer at an angle (.THETA.) with respect to the normal of the wafer of between zero degrees and a predetermined maximum angle. For the inspection system disclosed, the predetermined maximum angle is the angle formed when the beam of monochromatic light is as far away from the optic axis as possible yet still within the numerical aperture of the Fourier transform lens (16). Moreover, if a specific range of defect sizes is anticipated, the system can be optimized by setting the angle (.THETA.

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: Circuit protection devices which comprise two columnar electrodes and a conductive polymer element, at least a part of which is a PTC element. The device is so constructed that if a hot zone forms in the PTC element when current is passed through the device, it forms at a location away from the electrodes, thus increasing the useful life of the device. In one preferred embodiment, the conductive polymer element has an intermediate portion of increased resistance, thus causing the hot zone to be located at or near the intermediate portion. The intermediate portion may be of reduced size and/or be composed of conductive polymer of relatively high resistivity.