Pattern inspection method and its apparatus

Abstract

In a pattern inspection apparatus for comparing images of corresponding areas of two patterns, which are formed so as to be identical, so as to judge that a non-coincident part of the images is a defect, the influence of unevenness in brightness of patterns caused by a difference of thickness or the like is reduced, whereby highly sensitive pattern inspection is realized. In addition, high-speed pattern inspection can be carried out without changing the image comparison algorithm. For this purpose, the pattern inspection apparatus operates to perform comparison processing of images in parallel in plural areas. Further, the pattern inspection apparatus operates to convert gradation of an image signal among compared images using different plural processing units such that, even in the case in which a difference of brightness occurs in an identical pattern among images, a defect can be detected correctly.

1 . An inspection method, comprising the steps of:
 illuminating a light on a specimen on which plural chips having identical patterns are formed;   imaging corresponding areas of two chips formed on the specimen to obtain inspection images and reference images with an image sensor; and   processing the obtained inspection image and the reference image to produce a difference image which indicates a difference between the inspection image and the reference image and detect a defect by comparing the difference image with a threshold,   wherein a threshold applied to a difference image which is produced by comparing the inspection image and the reference image obtained by imaging peripheral portion of the specimen is different from a threshold applied to a difference image which is produced by comparing the inspection image and the reference image obtained by imaging central portion of the specimen.   
   
   
       2 . An inspection method according to  claim 1 , wherein said threshold applied to a difference image which is produced by comparing the reference image obtained by imaging peripheral portion of the specimen is higher than said threshold applied to a difference image which is produced by comparing the inspection image and the reference image obtained by imaging central portion of the specimen. 
   
   
       3 . An apparatus for inspecting a pattern, comprising:
 a stage on which a specimen to be inspected is mounted, and movable at least in one direction;   an illuminator which illuminates a light on the specimen on which plural chips having identical patterns are formed;   an imaging unit which forms and captures an image of corresponding areas of two chips formed on the specimen to obtain inspection images and reference images; and   a processing unit which processes the obtained inspection image and the reference image to produce a difference image which indicates a difference between the inspection image and the reference image and detect a defect by comparing the difference image with a threshold,   wherein said processing unit further including a threshold value estimator to estimate a value of said threshold and a threshold applied to a difference image which is produced by comparing the inspection image and the reference image obtained by imaging peripheral portion of the specimen is different from a threshold applied to a difference image which is produced by comparing the inspection image and the reference image obtained by imaging central portion of the specimen.   
   
   
       4 . An apparatus for inspecting a pattern according to  claim 3 , wherein said threshold value estimator estimates a value of said threshold so that a threshold applied to a difference image which is produced by comparing the inspection image and the reference image obtained by imaging peripheral portion of the specimen is higher than a threshold applied to a difference image which is produced by comparing the inspection image and the reference image obtained by imaging central portion of the specimen.