Television – Special applications – Flaw detector
Reexamination Certificate
1999-01-19
2002-03-12
Diep, Nhon T (Department: 2613)
Television
Special applications
Flaw detector
C348S095000, C382S151000
Reexamination Certificate
active
06356300
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an automatic visual inspection apparatus, an automatic visual inspection method and a recording medium having recorded the automatic visual inspection program. More particularly, the invention relates to an automatic visual inspection technique adapted for the purpose of detecting the shape defect of the pattern in the photomask or reticule for lithography of a semiconductor device.
2. Description of the Related Art
In general, in the inspection step in the semiconductor manufacturing, the shape defect of the pattern in the photomask or reticule for lithography of a semiconductor device is inspected by using an image.
As the representative procedures for detecting the shape defect of the pattern by using an image, there are die-to-die system for making comparison by the adjacent chip and the template matting and the die to database system for forming a reference image by the CAD data and making comparison with it, as disclosed in U.S. Pat. No. 4,805,123, Feb. 14, 1989 of Donald F. Specht et al., “Automatic Photomask and Reticule Inspection Method and Apparatus including Improved Defect Detector and Alignment Sub-Systems”. As these procedures are simple and relatively small calculation amount, similar procedures are adopted in various automatic visual inspection apparatuses.
Japanese Patent Application Laid-Open No. 7-63691 discloses a pattern defect inspection method and its apparatus for detecting the pattern defect of the member to be inspected by detecting an edge direction in the reference image data for detecting the pattern defect of the an object to be inspected, applying the differential processing respectively to the reference image data and the inspection image data obtained by picking up the image of the object to be inspected according to the edge direction, comparing the reference image data which has been subjected to the differential processing and the inspection image data, and detecting the pattern defect from the difference of the image data between them.
According to the procedure of Donald F. Specht et al., when, for example, there is a defect of lacking in a corner in the inspection image as shown in
FIG. 1
, alignment is carried out by a template matching with the reference image of
FIG. 1B
, there may be a case where the images are overlaid together to make the edge parts agree as in
FIG. 1C and a
case where the images are overlaid together with the edge parts displaced as in FIG.
1
D. In a case that the images are overlaid together with full agreement of the edge parts, the difference of the two images are only in the corner in which the defect actually exists, and no false defect occurs. However, when the images are overlaid with displacement of the edge parts as in
FIG. 1D
, false defects occur in the edge part.
Further, as shown in
FIG. 2A
(a graph showing the variation of intensity of the image to be inspected) and in
FIG. 2B
(a graph showing the variation of intensity of the reference image), in a case that the variation of the intensity of the edge part differs between the inspection image and the reference image due to the difference of manufacturing step or image pickup conditions, a false defect occurs in making comparison between the two images as shown in FIG.
2
C.
Furthermore, as shown in
FIG. 3A
(a graph showing the variation of intensity of the image to be inspected) and in
FIG. 3B
(a graph showing the variation of intensity of the reference image), in a case that a difference arises in the size of offset between the image to be inspected and the reference image, or, as shown in
FIG. 4A
(a graph showing the variation of intensity of the image to be inspected) and in
FIG. 4B
(a graph showing the variation of intensity of the reference image), in a case that a difference occurs in the intensity under effect of the difference of gains of the detector or the intensity of the light source, there occurs a false defect.
Furthermore, according to the procedure of Donald F. Specht, et. al., in aligning the positions of the images to be compared, a intensity between the pixels is interpolated to one image to form an image with displacement of a sub-pixel in position, and alignment of a sub-pixel precision was made. Because of this, in a case that of making alignment in high precision, it is necessary to produce a large quantity of the images having delicate degrees of the position displacements for comparison, so that there has been a problem to require an enormous amount of calculations and memory capacity for the purpose.
On the other hand, according to the pattern defect inspection method and its apparatus as disclosed in Japanese Patent Application Laid-Open No. 7-673691, it is told that, even if there are some displacements of coordinates between the coordinate of the inspection image data of the inspection pattern and the reference image data of the reference pattern, it is possible to prevent generation of false defect and to detect securely the pattern defect only. However, the publication does not show the technical matters corresponding to the present invention as to the method of alignment of the images nor any description suggestive of it.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an automatic visual inspection apparatus, an automatic visual inspection method and a recording medium having recorded the automatic visual inspection program with which generation of false defect is prevented and there is no increase in the necessary calculation amount or memory capacity even when the precision of alignment is elevated.
In the automatic visual inspection apparatus of the present invention, an image division differential unit inputs an inspection image on the object to be inspected and a reference image for making comparison with the inspection image, and spatially differentiating the respective inputted images and dividing them into the predetermined number of divisions to form the divisional differential images. A pixel precision alignment unit overlays the inspection image on the reference image by carrying out the position alignment in pixel precision which is a pixel size precision between the inspection image and the reference image on each of the divisional differential image formed by the image division differential unit. A function fitting unit regards the point where the absolute pixel value is larger than the predetermined threshold value to be an edge in each divisional differential image, obtaining the direction of the edge, obtaining the distribution of the intensity of the absolute pixel value in the direction orthogonal with the edge, fitting a predetermined single peak function to it, obtaining the maximum value point which is a coordinate of the point where its function takes the maximum value by a sub-pixel precision which is the precision lower than the pixel size, and employing the resulting value as coordinate of an edge. A sub-pixel precision position displacement computing unit obtains a position displacement of the sub-pixel precision. After the laying over of the images together in the pixel precision position aligning unit, if, there is an edge in the region on the reference image corresponding to the neighborhood of the edge, these two edges are regarded as being in the corresponding relations, and said sub-pixel precision position displacement computing unit computes the difference of the mutual edge positions as a position displacement of the sub-pixel precision. An adjusted differential image forming unit forms the adjusted differential image by rewriting the pixel value so that the intensity profile of the pixel value in the direction orthogonal with the edge should follow the previously given single peak type function form on the corresponding two edges. An image comparison unit detects the defect of the object to be inspected by comparing the adjusted divisional differential image of the inspected image with the adjusted divisional differential image of the reference image b
Diep Nhon T
Foley & Lardner
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