System and method for selection of a reference die

Abstract

A system for selecting reference die images, such as for use with a visual die inspection system, is provided. The system includes a die image comparator, which compares a first die image to a second die image in order to create a difference image that contains only the differences between the two die images. The system also includes a difference image analysis system that receives data from the die image comparator. The difference image analysis system analyzes the difference image and determines whether there are any features of the difference image that indicate that either the first die image or the second die image should not be used as a reference die image.

What is claimed is:  
     
         1 . A system for selection of a reference die image comprising: 
 a die image comparator operable to create a difference image based upon a first die image and a second die image; and    a difference image analysis system coupled to the die image comparator, the difference image analysis system operable to analyze the difference image and to determine whether the first die image and the second die image may each be used as the reference die image.    
     
     
         2 . The system of    claim 1    further comprising a die imaging system coupled to the die image comparator, the die imaging system operable to create a digital representation of a die.  
     
     
         3 . The system of    claim 1    further comprising a die image storage system coupled to the die image comparator, the die image storage system operable to store data representative of the first die image and the second die image.  
     
     
         4 . The system of    claim 1    wherein the difference image analysis system further comprises a slope detector, the slope detector operable to determine whether the slope of a histogram changes from negative to positive.  
     
     
         5 . The system of    claim 1    wherein the difference image analysis system further comprises a size detector, the size detector operable to determine whether a size of an anomalous region exceeds a predetermined allowable size.  
     
     
         6 . The system of    claim 1    wherein the difference image analysis system further comprises a density detector, the density detector operable to determine whether a number of anomalous regions per unit area exceeds a predetermined allowable number of anomalous regions per unit area.  
     
     
         7 . A system for inspecting dies comprising: 
 a camera configured to obtain an image of one or more dies; and    a reference die detection system coupled to the camera, the reference die detection system operable to determine whether a first die image and a second die image may be used as reference images.    
     
     
         8 . The system of    claim 7    wherein the reference die detection system further comprises an image comparator operable to produce a difference image from the first die image and the second die image.  
     
     
         9 . The system of    claim 8    wherein the reference die detection system further comprises a difference analyzer coupled to the image comparator, the difference analyzer operable to determine whether the difference image contains unacceptable features.  
     
     
         10 . The system of    claim 9    wherein the difference analyzer further comprises a data sorter that is operable to receive brightness data associated with a plurality of pixels of the difference image and to create a histogram from the brightness data.  
     
     
         11 . The system of    claim 10    wherein the difference analyzer further comprises a slope detector coupled to the data sorter, the slope detector operable to determine whether a slope of the brightness data histogram changes from negative to positive as a brightness magnitude increases.  
     
     
         12 . The system of    claim 10    wherein the difference analyzer further comprises a dimension analyzer that is operable to determine (a) one or more dimensions for a group of pixels, where each pixel has a brightness magnitude that exceeds a predetermined allowable magnitude, and (b) whether one or more dimensions of the group of pixels exceeds one or more predetermined allowable dimensions.  
     
     
         13 . The system of    claim 10    wherein the difference analyzer further comprises a density analyzer that is operable to determine (a) one or more dimensions of two or more groups of pixels, where each group of pixels has a brightness magnitude that exceeds a predetermined allowable magnitude, and (b) whether a density of the two or more groups of pixels per unit area exceeds a predetermined allowable density.  
     
     
         14 . The system of    claim 9    wherein the difference analyzer further comprises a data sorter that is operable to receive image data associated with a plurality of pixels of the difference image and to create a histogram from the image data.  
     
     
         15 . The system of    claim 14    wherein the difference analyzer further comprises a slope detector coupled to the data sorter, the slope detector operable to determine whether a slope of the image data histogram changes from negative to positive as an image data magnitude increases.  
     
     
         16 . The system of    claim 14    wherein the difference analyzer further comprises a dimension analyzer that is operable to determine (a) one or more dimensions of a group of pixels, where each group of pixels has an image data magnitude that exceeds a predetermined allowable magnitude, and (b) whether the dimensions of the group of pixels per unit area exceeds one or more predetermined allowable dimensions.  
     
     
         17 . The system of    claim 14    wherein the difference analyzer further comprises a density analyzer that is operable to determine (a) one or more dimensions of two or more groups of pixels, where each group of pixels has an image data magnitude that exceeds a predetermined allowable magnitude, and (b) whether a density of the two or more groups of pixels per unit area exceeds a predetermined allowable density.  
     
     
         18 . A method for selecting a reference die image comprising: 
 subtracting a first die image from a second die image to create a difference image; and    determining whether the difference image contains unacceptable data.    
     
     
         19 . The method of    claim 18    wherein subtracting the first die image from the second die image comprises subtracting brightness data for each pixel of the first die image from brightness data for a corresponding pixel of the second die image.  
     
     
         20 . The method of    claim 18    wherein subtracting the first die image from the second die image comprises subtracting other image data for each pixel of the first die image from other image data for a corresponding pixel of the second die image.  
     
     
         21 . The method of    claim 18    wherein determining whether the difference image contains unacceptable data comprises: 
 forming a histogram from difference image data; and  
 determining whether a slope of the histogram changes from negative to positive.  
 
     
     
         22 . The method of    claim 18    wherein determining whether the difference image contains unacceptable data comprises determining whether a size of an area having a brightness deviation exceeds a predetermined allowable size.  
     
     
         23 . The method of    claim 18    wherein determining whether the difference image contains unacceptable data comprises determining whether a size of an area having an other image data deviation exceeds a predetermined allowable size.  
     
     
         24 . The method of    claim 18    wherein determining whether the difference image contains unacceptable data comprises determining whether a number of areas having brightness deviations exceeds a predetermined allowable number of areas having brightness deviations per unit area.  
     
     
         25 . The method of    claim 18    wherein determining whether the difference image contains unacceptable data comprises determining whether a number of areas having other image data deviations exceeds a predetermined allowable number of areas having other image data deviations per unit area.  
     
     
         26 . The method of    claim 18    further comprising: 
 selecting two or more difference images, where each difference image is selected from a different predetermined region of the silicon wafer; and  
 combining the two or more difference images to form a reference image for use in comparing with each die of the silicon wafer.