Optics: measuring and testing – Inspection of flaws or impurities – Surface condition
Reexamination Certificate
2000-05-26
2004-08-10
Smith, Zandra V. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
Surface condition
C356S237500, C250S559410, C250S559450
Reexamination Certificate
active
06774991
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the inspection of semiconductor wafers and more particularly, to a method and apparatus for inspecting a surface of a semiconductor wafer having repetitive patterns for pattern defects and/or contaminant particles.
Integrated circuits (ICs) are commonly manufactured through a series of processing steps. Very often more than a hundred processing steps are performed to produce a properly functioning integrated circuit chip.
A semiconductor material, commonly in the shape of a wafer, serves as the substrate for integrated circuits. Semiconductor ICs are typically manufactured as an assembly of a hundred or more chips on a single semiconductor wafer, which is then cut up to produce the individual IC chips. Typically, a wafer made of silicon is used as the integrated circuit substrate, the silicon wafer being approximately 150-200 mm in diameter and 0.5-1 mm thick. During the manufacturing process, the silicon wafer is first polished and cleaned to remove any contaminant particles situated thereon. The silicon wafer is then is treated in preparation for a series of processing steps involving a plurality of photolithographic patterns (also commonly referred to as masks). In the production of integrated circuits, microelectronic circuits are formed onto the silicon wafer through a process of layering. In the layering process, conductive and insulative layers of thin films are deposited and patterned onto the silicon wafer. Each layer is patterned by a mask designed specifically for it, the mask defining the areas within the wafer that are to be treated such as by etching or implanting.
Semiconductor fabrication technology today deals with silicon wafers which are approximately 200 mm in diameter and which feature geometries with dimensions well below 1 &mgr;m (micrometer). Due to the high complexity and level of integration of integrated circuits, the absence of contaminants on every layer of the wafer is critical in order to realize acceptable levels of product yield. However, it has been found that contaminant particles are often introduced onto the semiconductor wafer during the manufacturing process of integrated circuits. As a consequence, the presence of one contaminant particle larger than the half the width of a conductive line on the silicon wafer can result in complete failure of a semiconductor chip produced from the wafer. Such a wafer has to be discarded which thereby decreases the percentage yield per wafer and increases the overall cost of the individual chips. Therefore, a critical task facing semiconductor process engineers is to identify and, as far as possible, to eliminate sources of surface contamination on each layer of the semiconductor wafer.
Accordingly, inspection systems are well known in the art and are commonly used to detect, view, identify and correct yield limiting defects introduced in the fabrication process of integrated circuits. Wafer inspection systems often include a light source, such as a laser, and a light sensitive imaging camera, or detector. In use, the light source is used to scan the surface of the wafer by means of illuminating particular regions of the surface of the wafer. The light sensitive imaging camera is positioned relative to the wafer to pick up scattered light for display on a viewing screen for further analysis. The imaging camera creates a visual for the viewing screen based on the number of photons which disperse from the wafer as the laser performs its scanning function. The visual could equivalently be formed by use of a non-imaging detector (e.g. a photomultiplier tube) with appropriate means of scanning for image formation. The camera will detect light scattered from any contaminant particles situated on the wafer, the intensity of the scattered light being generally proportional to the size of the particles, wherein the larger particles generally reflect more photons onto the imaging camera than smaller particles. As a consequence, larger particles will produce a brighter image and will have a greater light intensity than smaller particles.
Inspection systems of the type described above have been made commercially available by such companies as Inspex, Inc. of Billerica, Mass.
In U.S. Pat. No. 4,772,126 to C. D. Allemand et al, there is disclosed an apparatus and method for detecting the presence of particles on the surface of an object such the front side of a patterned semiconductor wafer. A vertically expanded, horizontally scanning, beam of light is directed onto an area on the surface of the object at a grazing angle of incidence. A video camera positioned above the surface detects light scattered from any particles which may be present on the surface, but not specularly reflected light. The surface is angularly prepositioned (rotated) relative to the incident light beam so that the diffracted light from the surface and the pattern of lines on the surface is at a minimum. The object is then moved translationally to expose another area to the incident light beam so that the entire surface of the object or selected portions thereof can be examined, an area at a time.
In U.S. Pat. No. 4,895,446 to M. A. Maldari et al. an apparatus and method are disclosed for detecting the presence of particles on the surface of an object such as the front side of a patterned semiconductor wafer. A collimated beam of light is directed onto an area on the surface of the object at a grazing angle of incidence. A detector positioned above the surface detects light scattered from any particles which may be present on the surface, but not specularly reflected light. The output of the detector is fed into a computer where the information is processed and then displayed on a display. The surface is prepositioned relative to the incident light beam so that the diffracted light from the surface and the pattern on the surface is at a minimum. The object is then moved translationally too expose another area to the incident light beam so that the entire surface of the object or selected portions thereof can be examined, an area at a time.
In U.S. Pat. No. 5,659,390 to J. J. Danko, there is disclosed a method and apparatus for detecting particles on a surface of a semiconductor wafer having repetitive patterns. The apparatus for detecting particles on the front surface of a patterned semiconductor wafer having repetitive patterns includes a laser for illuminating an area on the front surface at grazing angle of incidence with a beam of polarized light. A lens collects light scattered from the area and forms a Fourier diffraction pattern of the area illuminated. A Fourier mask blocks out light collected by the lens at locations in the Fourier diffraction pattern where the intensity is above a predetermined level indicative of background information and leaves in light at locations where the intensity is below the threshold level indicative of possible particle information. The Fourier mask includes an optically addressable spatial light modulator and a crossed polarizer with the Fourier diffraction pattern being used as both a read beam and a write beam for the spatial light modulator. A camera detects scattered light collected from the area by the lens and not blocked out by the Fourier mask.
Although widely used in commerce, inspection systems of the type described above have been found, on occasion, to be unsatisfactory in detecting the majority of notable particles disposed on the substrate. Rather, inspection systems of the type described above have been found, on occasion, to detect only a small fraction of the total number of notable defects on the substrate, which is highly undesirable.
The effectiveness in which an inspection system can detect particles on a substrate is dependent upon certain characteristics. As a first characteristic, the intensity of the detectable scattered light is dependent upon the physical and geometrical attributes of the defect, such as the size, shape, orientation and/or index of refraction of the particle. As a second characteristic, the
Inspex Incorporated
Kriegsman & Kriegsman
Smith Zandra V.
Stock, Jr. Gordon J.
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