Focusing method and apparatus

Abstract

Methods and apparatus for placing wafers axially in an optical inspection system. A “best worst” focus method includes a series of through-focus images of a test wafer acquired using full field of view of the inspection optics. The value of the worst quality in each image is associated with the respective axial location, forming a locus of “worst” values as a function of axial location. The axial location is chosen which optimizes the locus, giving an axial location that provides the “best-worst” image quality. A “video focus” method includes a series of through-focus images generated using reduced field of view. A figure of merit is associated with each image, providing through-focus information. The “video focus” can be calibrated against the “best worst” focus. Further, a point sensor can be used to generate a single z-value for one (x,y) location that can be calibrated with “video focus”.

1 . A method of determining a desired axial location for placement of an imageable object in an inspection optical system, comprising:
 placing an axially-translatable object within a field of view of the inspection optical system;   axially translating the object sequentially to a plurality of axial locations;   acquiring a plurality of images of the translated object, each of the plurality of images being associated with a respective axial location;   determining a plurality of lateral-location-dependent figures of merit for the plurality of images, each of the plurality of lateral-location-dependent figures of merit being associated with a respective axial location;   identifying a plurality of worst image quality values from the plurality of lateral-location-dependent figures of merit, each of the plurality of worst image quality values being associated with a respective axial location;   determining a best worst image quality value from the plurality of worst image quality values;   associating the best worst image quality value with a desired axial location, based on the associations between the worst image quality values and the axial locations; and   selecting the desired axial location.   
   
   
       2 . The method of  claim 1 , wherein each of the plurality of lateral-location-dependent figures of merit is an edge transition width. 
   
   
       3 . The method of  claim 2 , wherein each of the plurality of worst image quality values is a maximum edge transition width. 
   
   
       4 . The method of  claim 1 , wherein each of the plurality of lateral-location-dependent figures of merit is a contrast. 
   
   
       5 . The method of  claim 4 , wherein each of the plurality of worst image quality values is a minimum contrast. 
   
   
       6 . The method of  claim 1 , wherein the best worst image quality value is one of the plurality of worst image quality values. 
   
   
       7 . The method of  claim 6 , wherein the desired axial location is the axial location associated with the best worst image quality value. 
   
   
       8 . The method of  claim 1 , wherein the best worst image quality value is interpolated between two of the plurality of worst image quality values. 
   
   
       9 . The method of  claim 8 , wherein the desired axial location is interpolated between two of the plurality of axial locations. 
   
   
       10 . The method of  claim 1 , wherein the axially-translatable object has at least one imageable feature within the field of view of the inspection optical system. 
   
   
       11 . The method of  claim 1 , further comprising:
 placing a second axially-translatable object within the field of view of the inspection optical system;   defining a reduced field of view of the inspection optical system;   axially translating the second object sequentially to a second plurality of axial locations;   acquiring a second plurality of images of the second translated object, each of the second plurality of images being associated with a respective axial location, each image in the second plurality having the reduced field of view; and   selecting a second axial location based on the second plurality of images and based on a predetermined axial offset.   
   
   
       12 . The method of  claim 11 , wherein the predetermined axial offset accounts for effects within the full field of view of the inspection optical system but outside the reduced field of view of the inspection optical system. 
   
   
       13 . The method of  claim 11 , wherein the second axially-translatable object is different from the first axially-translatable object. 
   
   
       14 . The method of  claim 11 , further comprising:
 measuring a third axial location of a predetermined lateral location on the second axially-translatable object with a point sensor; and   calibrating the point sensor to account for a difference between the second and third axial locations.   
   
   
       15 . A method of calibrating an optical inspection system, comprising:
 calculating a “best worst” axial position for a first objective lens using a full field of view of the first objective lens;   acquiring a series of through-focus images using a reduced field of view of the first objective lens; and   calibrating the series of through-focus images to the “best worst” axial position.   
   
   
       16 . The method of  claim 15 , further comprising:
 axially translating a wafer through focus;   acquiring a series of through-focus images of the wafer using a reduced field of view of the first objective lens; and   placing the wafer at the “best worst” axial position, based on the series of through-focus images, and not based on acquisition of any images using the full field of view of the first objective lens.   
   
   
       17 . The method of  claim 15 , wherein the “best worst” axial position is obtained from analyzing a series of full field of view through-focus images using the full field of view of the first objective lens. 
   
   
       18 . The method of  claim 15 , wherein calibrating a “best worst” axial position comprises:
 assigning a video focus figure of merit to each of the series of through-focus images;   generating an association between the video focus figure of merit and axial position based on the series of through-focus images; and   selecting a video focus figure of merit corresponding to the “best worst” axial position.   
   
   
       19 . The method of  claim 15 , further comprising:
 measuring a point sensor axial location with a point sensor; and   calibrating the series of through-focus images to the point sensor axial position.   
   
   
       20 . The method of  claim 15 , further comprising:
 repeating for a predetermined length of time:
 axially positioning and inspecting each of a series of wafers, the positioning being performed with a point sensor, the inspecting being performed using the full field of view of the objective lens; and 
 calibrating the point sensor to the series of through-focus images. 
   
   
   
       21 . The method of  claim 15 , further comprising:
 repeating for a predetermined number of wafers:
 axially positioning and inspecting each of a series of wafers, the positioning being performed with a point sensor, the inspecting being performed using the full field of view of the objective lens; and 
 calibrating the point sensor to the series of through-focus images. 
   
   
   
       22 . The method of  claim 15 , further comprising:
 repeating for a predetermined number of inspections:
 axially positioning and inspecting each of a series of wafers, the positioning being performed with a point sensor, the inspecting being performed using the full field of view of the objective lens; and 
 calibrating the point sensor to the series of through-focus images. 
   
   
   
       23 . The method of  claim 15 , further comprising:
 repeating when an accelerometer disposed on the inspection system is triggered:
 axially positioning and inspecting each of a series of wafers, the positioning being performed with a point sensor, the inspecting being performed using the full field of view of the objective lens; and 
 calibrating the point sensor to the series of through-focus images. 
   
   
   
       24 . A method of inspecting a substrate, comprising:
 determining a “best worst” axial location;   acquiring a series of reduced-field-of-view, through-focus images of a substrate;   correlating the reduced-field-of-view, through-focus images to the “best worst” axial location; and   placing the substrate at the “best worst” axial location.   
   
   
       25 . The method of  claim 24 , wherein determining a “best worst” axial location comprises:
 acquiring a series of full-field-of-view, through-focus images of a test object;   determining a through-focus worst image quality from the full-field-of-view, through-focus images;   selecting the best worst image quality from the through-focus worst image quality; and   selecting an axial location corresponding to the best worst image quality to be the “best worst” axial location.   
   
   
       26 . The method of  claim 24 , further comprising:
 measuring a point sensor axial location on the substrate with a point sensor; and   calibrating the point sensor axial position to the reduced-field-of-view, through-focus images.