The present invention relates generally to the inspection of semiconductor integrated circuit (IC) chips and more particularly, to a method and apparatus for inspecting a semiconductor wafer using a dynamic threshold.
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 all 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 .mu.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 typically include a light source, such as a laser, and a light sensitive imaging camera. 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. For example, 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 are 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.
Inspection systems of the type described above typically establish a single, constant, minimum light intensity threshold level for the entire wafer. The minimum light intensity threshold level is established to filter out the levels of photon dispersion which fall below the threshold level from further identification and analysis. This enables the wafer inspection system to limit its detection, identification and analysis to the larger defects on the wafer which may prove to be yield limiting and accordingly filters out background noise and the relatively small particles which would typically be found not to be yield limiting. This enables the wafer inspection system to operate with increased overall inspection speed, system throughput and efficiency.
However, the establishment of a single, constant threshold level for the entire wafer has its drawbacks. Specifically, it has been found that certain conditions may cause the semiconductor wafer to appear on the imaging camera as darker in one region or lighter in another region. This regional light intensity differential could be attributed to a non-uniform or non-level layer of material being deposited on the wafer during the manufacturing process. The regional light intensity differential could also be caused by other conditions, such as if the wafer is warped or if a platform on which the wafer is placed is not perfectly level.
Due to the variances in light intensity of different regions on the wafer, a particle located in a darker region of the wafer will appear to have a relatively lower level light intensity than if that same particle were located in a lighter region. As a consequence, it has been found that large and potentially yield limiting particles which are located in darker regions of the wafer will often produce a light intensity that is below the threshold value and therefore will not be identified and analyzed. This lowers the wafer inspection system sensitivity level to the point that many large defects worthy of identification are filtered out and ignored from further analysis.
Furthermore, it has been found that small, non-yield limiting particles which are located in lighter regions of the wafer will often produce a light intensity that is above the threshold value and therefore will be detected and identified for further analysis. This creates a wafer inspection system which is oversensitive. Oversensitivity of a wafer inspection system causes many small defects which are not yield limiting to be identified and analyzed. This, in turn, decreases the speed in which the inspection system can analyze an entire wafer, thereby limiting its productivity.