Optics: measuring and testing – Inspection of flaws or impurities – Surface condition
Reexamination Certificate
1998-12-16
2001-04-10
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
Surface condition
Reexamination Certificate
active
06215551
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates in general to surface inspection systems, and in particular, to a high speed scanner system for inspecting anamolies on surfaces such as semiconductor wafers, photomasks, reticles, ceramic tiles, and other surfaces.
The size of semiconductor devices fabricated on silicon wafers has been continually reduced. At the time this application is filed, for example, semiconductor devices can be fabricated at a resolution of a half micron or less and sixty-four (64) megabyte DRAMs are being fabricated with 0.35 micron design rule. The shrinking of semiconductor devices to smaller and smaller sizes has imposed a much more stringent requirement for sensitivity of wafer inspection instruments which are called upon to detect contaminant particles and pattern defects that are small compared to the size of the semiconductor devices. On the other hand, it is desirable for wafer inspection systems to provide an adequate throughput so that these systems can be used in production runs to detect defective wafers.
In U.S. Pat. No. 4,898,471 to Stonestrom et al. assigned to the present assignee of this application, the area illuminated on a wafer surface by a scanning beam is an ellipse which moves in the scan direction. In one example given by Stonestrom et al., the ellipse has a width of 20 microns and a length of 115 microns. Light scattered by anamolies or patterns in such illuminated area is detected by photodetectors placed at azimuthal angles in the range of 80 to 100°. The signals detected by the photodetectors are used to construct templates. When the elliptical spot is moved in the scan direction to a neighboring position, scattered light from structures within the spot is again detected and the photodetector signal is then compared to the template to ascertain the presence of contaminant particles or pattern defects as opposed to regular pattern. In Stonestrom et al., the scanning beam scans across the entire wafer to illuminate and inspect a long narrow strip of the wafer extending across the entire dimension of the wafer in the scanning direction. The wafer is then moved by a mechanical stage in a direction perpendicular to the scanning direction for scanning a neighboring elongated strip. This operation is then repeated until the entire wafer is covered.
While the system of Stonestrom et al. performs well for inspecting wafers having semiconductor devices that are fabricated with coarser resolution, with the continual shrinking of the size of the devices fabricated, it is now desirable to provide an improved inspection tool that can be used to detect very small size anamolies that may be difficult to detect using Stonestrom et al.'s system.
SUMMARY OF THE INVENTION
This invention is based on the recognition that very small anamolies can be detected by reducing the size of the area that is illuminated by the scanning light beam. Light scattered from structures in the spot will include background, such as light scattered by pattern on the surface, as well as light that is scattered by anomalies such as contaminant particles, pattern defects or imperfections of the surface. Such background can have a significant amplitude. For this reason, if the anamoly is of a size which is small compared to the size of the illuminated area, the scattered light from such anamoly may be overwhelmed by and become undetectable from the background. By reducing the size of the illuminated area or spot size, the ratio of the light intensity scattered by an anomaly to that of the background will be increased, thereby increasing detection sensitivity. However, with a smaller spot size, it will be more difficult to maintain the uniformity of the spot along a long straight scan line across the entire wafer. By breaking up the scan path into short segments, it is possible to employ a smaller spot size while at the same time maintaining uniformity of the spot along the path. From the system point of view, by reducing the length of the scan, the size of the collection optics for detecting forward scattered light becomes more manageable.
Thus one aspect of the invention is directed towards a method for detecting anamolies on a surface, comprising the steps of directing a beam of light at a grazing angle towards the surface, causing relative motion between the beam and the surface so that the beam scans a scan path covering substantially the entire surface; and collecting light scattered along said path for detecting anamolies. The scan path includes a plurality of arrays of scan path segments, wherein each of at least some of such scan path segments has a span shorter than the dimensions of the surface.
As used in this application, “minimum width” of the illuminated area or spot on the surface to be inspected is defined as the minimum dimension of a boundary around the area or spot along any direction on the surface, where the boundary is defined as the locations on the surface where the illumination light intensity is a predetermined fraction or percentage of the maximum intensity of illumination in the area or spot. In the description of the preferred embodiment, for example, the boundary is where the light illumination intensity is l/e
2
of the maximum intensity of illumination in the area or spot, e being the natural number. The minimum dimension is the minimum distance between two parallel lines that enclose between them the boundary of the area or spot. The term “minimum width” is explained in more detail below.
Another consideration of the invention is to provide an adequate throughput while data is collected at a moderate rate for defect detection so that the data collection and processing system employed need not be overly complex and expensive.
Thus another aspect of the invention is directed towards a method for detecting anamolies on the surface of a semiconductor wafer, comprising directing a beam of light towards a surface to illuminate an area of the surface defining a spot having a spot size whose minimum width is in the range of about 5 to 15 microns, causing relative motion between the beam and the wafer so that the beam scans a path covering the entire surface; and collecting light scattered along said path for detecting anamolies. The spot size and the directing and causing steps are such that the beam scanning substantially inspects the entire surface of the wafer at a throughput in excess of about 40 wafers per hour for 150 millimeter diameter wafers, at a throughput in excess of about 20 wafers per hour for 200 millimeter diameter wafers, and at a throughput in excess of about 10 wafers per hour for 300 millimeter diameter wafers.
Yet another aspect of the invention is directed towards a method for detecting anamolies on a surface, comprising the steps of directing a beam of light towards the surface to illuminate an area of the surface defining a spot having a spot size whose minimum width is in the range of about 5 to 15 microns, causing relative motion between the beam and the surface so that the beam scans a path covering substantially the entire surface; and collecting light scattered along said path for detecting anamolies. The spot size and said directing and causing steps are such that the surface is inspected at a speed not less than about 1.5 cm
2
/s.
Still another aspect of the invention is directed towards a method for detecting anamolies on a surface, comprising the steps of directing a beam of light towards said surface to illuminate an area of the surface defining a spot having a spot size whose minimum width is in the range of about 5 to 15 microns, causing relative motion between the beam and the surface so that the beam scans a path covering substantially the entire surface; and collecting light scattered along said path for detecting anamolies. The surface has dimensions of not less than 200 millimeters in any direction along the surface. The directing and causing steps are such that the beam scans substantially the entire surface in about 50 to 90 seconds.
Another aspect of the invention is directed towards a system
Nikoonahad Mehrdad
Stokowski Stanley E.
Kla-Tencor Corporation
Rosenberger Richard A.
Skjerven Morrill & MacPherson LLP
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