Automatic wafer alignment technique
First Claim
1. A method for use in a system in which a video camera is used to view a target of expected characteristics, said method being used for ascertaining whether a feature evident in the video image corresponds to the actual target, said video camera providing output data consisting of an average intensity value for each scan line or column of said video image, comprising:
- ascertaining, from said video data, each feature edge evident in the video display,sequentially correlating the data for each ascertained edge with the data for each other ascertained edge to determine all pairs of edges which represent potential targets, andselecting the best target by weighted comparison of selected parameters of all determined pairs of edges.
3 Assignments
0 Petitions
Accused Products
Abstract
This automatic wafer alignment technique may be used in a step-and-repeat photomicrolithographic exposure system accurately to prealign a wafer before exposure to the B-level and subsequent reticles, and automatically to align the wafer at each die site prior to exposure. A search technique optimizes location of the global targets used for wafer prealignment. The search begins at the most likely target position and proceeds through a search area established by the maximum expected rough prealignment error. A wafer alignment target configuration consisting of a cross with one elongated arm and a short crossbar is used to optimize target verification. To locate targets a video image is digitized and the average intensity at each video scan line and column is obtained and stored. This data is used to determine the presence in the video image of feature edges. A table of all such edges is established. These listed edges are correlated to determine all pairs of edges that may represent potential targets. From these, the best target is selected by comparing certain weighted parameters of each potential target.
-
Citations
11 Claims
-
1. A method for use in a system in which a video camera is used to view a target of expected characteristics, said method being used for ascertaining whether a feature evident in the video image corresponds to the actual target, said video camera providing output data consisting of an average intensity value for each scan line or column of said video image, comprising:
-
ascertaining, from said video data, each feature edge evident in the video display, sequentially correlating the data for each ascertained edge with the data for each other ascertained edge to determine all pairs of edges which represent potential targets, and selecting the best target by weighted comparison of selected parameters of all determined pairs of edges. - View Dependent Claims (2, 3, 4, 5, 6)
-
-
7. A method for determing the location of edges of features evident in a video image sensed by a video camera, comprising:
-
obtaining for each video scan line or column a digital value representing the average intensity along such scan line or column, comparing, for each such scan line or column, the intensity value of that scan line or column with the intensity value of each the next two successive scan lines or columns, thereby to obtain a first intensity difference between said each scan line or column and the next one following and a second intensity difference between said each scan line or column and the second next following one, determining from (a) the intensity difference between said each scan line or column and the next preceding scan line or column, and from (b) said first and second intensity differences, that the preceding, each and next scan lines or columns belong to a potential edge, by using the criteria that, for a potential edge, the signs of such preceding and first intensity differences are the same, or that said first intensity difference is zero and the signs of the said preceding and second intensity differences are the same, and determining the end point of such edge by the criteria that an edge ends when two consecutive intensity differences are zero, or that the sign of one intensity difference is the opposite of the sign of the preceding two intensity differences.
-
-
8. A method for use in a wafer alignment system employed with a wafer processing apparatus in which a semiconductor wafer is to be aligned in a certain orientation with respect to the reference axes of said system, said wafer containing at least one alignment target designated as a global alignment target, the wafer being roughly prealigned to within certain rotational error and lateral offset error tolerances, and in which a small region of the wafer is viewed by a video camera for alignment purposes, the method being used for searching for said global target, comprising:
-
first positioning said wafer so that said video camera views a first area thereon at which said global target would be located if the wafer were correctly positioned, and determining from the output of said video camera if the global target is visible at said area, thereafter moving said wafer alternately in one direction and the opposite direction along the system axis closest to that along which the target would be displaced as a result of rotational error in rough positioning, by amounts increasing in each such direction by steps corresponding to the dimension in that direction of the region viewed by said video camera, such being repeated until the distance searched in each direction from said first area is substantially equal to the expected maximum offset of said target which would result if the rough alignment were to have said maximum expected rotational error, and determining from said vidio camera output, at each such location, if said global target is visible at such location, and thereafter repeating such moving and determining first along one then along another of two paths parallel to the initial movement path but laterally offset therefrom by respective positive and negative amounts corresponding to the expected maximum lateral offset error of said wafer, and terminating such search at the location at which said alignment target is visible by said video camera. - View Dependent Claims (9)
-
-
10. A method for use in an apparatus requiring alignment of one article, said one article containing an alignment target that is viewed by a video camers, said method being used for verication that a feature in the image being viewed by said video camera is actually said alignment target, comprising:
-
utilizing for the shape of said target a cross having a relatively elongated arm and a relatively short crossbar near the middle of said elongated arm, initially positioning said one article so that the image viewed by said video camera is near said middle and includes part of said elongated arm and part of said crossbar, and, at such position, verifying from the output of said video camera the presence in said image of both elongated arm part and said crossbar part, and thereafter successively moving said one article to positions at which the image viewed by said video camera would be expected to include only part of one end of said elongated arm, only part of the other end of said elongated arm, only the region beyond one end of said elongated arm and only the region beyond the other end of said elongated arm, and at each such position, verifying from the output of said video camera the expected presence or absence of said arm parts. - View Dependent Claims (11)
-
Specification