Image analysis – Image transformation or preprocessing – Measuring image properties
Reexamination Certificate
1995-03-14
2001-05-29
Bella, Matthew C. (Department: 2621)
Image analysis
Image transformation or preprocessing
Measuring image properties
C382S170000, C382S287000
Reexamination Certificate
active
06240218
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to machine vision, and particularly to automated visual pattern location.
BACKGROUND OF THE INVENTION
The process of making an integrated circuit from a silicon wafer employs photolithography, which includes the step of transferring geometric patterns, called features, to a photosensitive film applied to the surface of the wafer. Some of these features become the various circuit elements (transistors, resistors, capacitors, etc.) of the integrated circuit, and some features are used for alignment and registration.
A stepper, also called a stepping aligner, is an optical system that repeatedly projects a geometric pattern onto the photosensitive film-coated wafer over a sequence of positions on the surface of the wafer. After each projection of the geometric pattern, the wafer is moved, i.e., “stepped,” to a new position, and the projection is repeated.
The step of transferring a single geometric pattern at a position on the silicon wafer is called a masking step. During the fabrication of an integrated circuit, several masking steps are performed at each position, each masking step overlaying a different geometric pattern over the previous ones. The pattern that results from a sequence of masking steps is shown in
FIG. 1
, where the progression from a single mask pattern to a complex overlap of many geometric patterns is indicated by the arrow
18
.
Before each masking step, the pattern to be transferred to the wafer must be precisely sized and aligned with respect to the previous patterns. To accomplish this alignment, the surface of the wafer is visually inspected, typically using an optical device. However, each masking step changes the visual appearance of the wafer's surface. These changes in appearance commonly include distortion and degradation of fiducial marks or other reference features used to achieve alignment visually. Consequently, it is difficult to achieve accurate and repeatable visual alignment over a sequence of masking steps.
Further, known alignment techniques require that the reference features be clearly distinguishable with respect to background features and random noise. However, each photolithographic step can change the appearance of the reference features on the wafer to an extent that, after a sequence of masking steps, the reference feature becomes indistinguishable from background image noise. Moreover, a masking step can introduce a change in image polarity, which introduces further difficulty.
A known method for locating a reference feature is to perform a two-dimensional search using, for example, the Search Tool, as sold by Cognex Corporation, Natick, Mass. However, this method is impractical whenever the reference features of the object under inspection are degraded by one or more processing steps. Moreover, since each processing step has an unpredictable effect on the appearance of the object's features, is difficult to train a search model for use with more than one masking step. In addition, rotation may be introduced between masking steps, which a two-dimensional search tool typically cannot accommodate.
SUMMARY OF THE INVENTION
The invention is a method and apparatus for determining the location and orientation of a reference feature in an image. The invention includes two phases: a train-time phase and a run-time phase. At train-time, a template image is created for use during the run-time phase. At run-time, a reference feature is located by first determining the principal angle of the reference feature. The projected image is similar to the template image in that it also has the form of an X-&Sgr;I plot, i.e., a plot of summed pixel intensity as a function of displacement along an axis of a coordinate system. In a preferred embodiment, the principal angle &thgr; of the reference feature is found by partitioning the reference feature into a plurality of regions, and in a preferred embodiment, projecting each of the regions at 0° and performing template matching with a template image to determine the position of a plurality of points along a principal axis of the reference feature. Then, either an equation of a “best fit” line can be calculated and solved simultaneously with an equation of a horizontal reference line, or the reference feature image can be projected at the angle of the “best fit” line to obtain a one-dimensional reference image which can be matched to the one-dimensional template image to find the relative displacement between them that maximizes a match-metric value. Each way of exploiting the principal angle information of the “best fit” line results in a value for displacement along an axis that indicates the location of the reference feature image.
The invention is particularly useful for determining the location and orientation of reference feature images that have been degraded in some way, either by problems of lighting, variations in surface optical properties introduced by process steps, such as semiconductor processing steps, partial image occlusion, and video signal degradation, such as the introduction of video noise.
The invention provides substantial immunity to such image degradation by comprehensively exploiting image information derived from the entire image of the reference feature. The invention recognizes that it is advantageous to span three independent dimensions, i.e., time, length, and width, to extract an image signal with optimized signal-to-noise ratio.
In particular, by using video averaging, various instances of an image over time can be combined to provide an image with less noise than any constituent image.
By projecting a two-dimensional reference feature image to provide a one-dimensional image, image information of the entire length of the reference feature is consolidated, thereby providing a one-dimensional image that is more robust than the two-dimensional image.
Additionally, matching over the entire one-dimensional image exploits information over the entire width of the image.
REFERENCES:
patent: 4677301 (1987-06-01), Tanimoto et al.
patent: 5020006 (1991-05-01), Sporon-Fiedler
patent: 5493403 (1996-02-01), Nishi
patent: 5506918 (1996-04-01), Ishitani
patent: 5543921 (1996-08-01), Uzawa et al.
patent: 5627912 (1997-05-01), Matsumoto
Garakani Arman
Michael David
Bella Matthew C.
Cognex Corporation
Weinzimmer Russ
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