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Optical inspection method and apparatus    

An object of the technology is to provide a novel method and apparatus having advantages in the above respects for inspecting the surface of articles for defects.

Overview

A method and apparatus for inspecting the surface of articles, such as chips and wafers, for defects, includes a first phase of optically examining the complete surface of the article inspected at a relatively high speed and with a relatively low spatial resolution, and a second phase of optically examining with a relatively high spatial resolution only the suspected locations for the presence or absence of a defect therein.


Summary of Technology


In particular, an object of the technology is to provide a method and apparatus for automatically inspecting patterned semiconductor wafers characterized by a relatively high speed and relatively low rate of false alarms such that the patterned wafers may be tested while the wafers are in the production line to quickly enable the fabrication personnel to identify any process or equipment causing yield reduction, to receive fast feedback information after corrective actions, and to predict potential yield loss.


A further object of the technology is to provide an inspection method and apparatus which are capable of inspecting all the critical layers, and which supply data on defects caused by the presence of particles and defects in the patterns.


According to one aspect of the technology, there is provided a method of inspecting the surface of an article for defects by: optically examining, in a first phase examination, the complete surface of the article and electrically outputting information indicating locations on the article suspected of having defects; storing the suspected locations in a storage device; and, in a second phase examination, optically examining with high resolution only the suspected locations of the articles surface for determining the presence or absence of a defect in the suspected locations; characterized in that the first phase examination is effected by optically scanning the complete surface of the article at a high speed with an optical beam of small diameter. Thus, by selecting the diameter of the optical beam used in the first phase examination, the first phase examination may be made at any desired resolution, as compared to the second phase examination, according to the particular application.


According to further features of the technology, the first examining phase is effected by optically scanning the complete article surface to be inspected with a laser beam of small diameter; and the second examining phase is automatically effected immediately after the first phase by imaging only the suspected locations on an image converter which converts the image to electrical signals and then analyzes the electrical signals.


The surface of the article to be inspected includes a pattern, e.g., a patterned wafer used for producing a plurality of integrated-circuit dies or chips. The first examination phase is effected by making a comparison between the inspected pattern and another pattern, serving as a reference pattern, to identify locations on the inspected pattern wherein there are sufficient differences with respect to the reference pattern to indicate a high probability of a defect in the inspected pattern. The second examination phase is also effected by making a comparison between the inspected pattern and the reference pattern, to identify locations on the inspected pattern wherein the comparison shows sufficient differences with respect to the reference pattern to indicate the presence of a defect in the suspected location of the inspected pattern.


The reference pattern may be a pattern on another like article (e.g., die-to-die comparison), another like pattern on the same article (repetitive pattern comparison), or data stored in a database (die-to-database comparison).


It will thus be seen that the novel method of the technology primarily monitors changes in the defect density while maintaining a high throughput with a relatively low false alarm rate. Thus, the first examination is done at a relatively high speed and with a relatively low spatial resolution such as with a laser beam of small diameter to indicate only suspected locations having a high probability of a defect; and the second examination is done with a relatively high spatial resolution but only with respect to the suspected locations having a high probability of a defect. The sensitivity of the two phases may be adjusted according to the requirements for any particular application. Thus, where the application involves a relatively low number of defects, the sensitivity of the first examination phase may be increased by using a very small diameter laser beam to detect very small defects at a high speed but at the expense of an increased false alarm rate. However, since only relatively few suspected locations are examined in the second phase, the overall inspection can be effected relatively quickly to enable the fabrication personnel to identify defects caused by any process or equipment, and to immediately correct the cause for such defects.


The first examining phase is effected by generating a first flow of N different streams of data representing the pixels of different views of the inspected pattern unit; generating a second flow of N different streams of data representing the pixels of different views of the reference; and comparing the data of the first flow with the data of the second flow to provide an indication of the suspected locations of the inspected surface of the article having a high probability of a defect.


The pattern is based on a grid of angularly-spaced lines (e.g., 45.degree. spacing); and the N streams of data in each flow are generated by a circular array of light collectors. The light collectors are located to collect the light in regions midway between the angularly-spaced lines of the grid. Such an arrangement minimizes the amount of pattern-reflected light, collected by the light collectors; that is, such an arrangement does not see most of the pattern, except pattern irregularities, corners and curves.


Preferably, there are eight light collectors each located to collect the light in a region midway between each pair of the angularly-spaced lines of the grid; it is contemplated, however, that the system could include another member, e.g., four such light collectors equally spaced between the grid lines.


The second examining phase is effected by imaging on a converter each suspected location of the inspected pattern unit and the corresponding location of the reference pattern unit to output two sets of electrical signals corresponding to the pixels of the inspected pattern unit and the reference pattern unit, respectively; and comparing the pixels of the inspected pattern unit with the corresponding pixels of the reference pattern unit to indicate a defect whenever a mismatch of a predetermined magnitude is found to exist at the respective location. Each suspected location of the inspected pattern unit and the reference pattern unit is imaged at a plurality of different depths, and the electric signals of one set are shifted with respect to those of the other set to match the respective depths of the images.


The technology also provides apparatus for inspecting articles, particularly patterned semiconductor wafers, in accordance with the above method.

Patent Summary

U.S. Patent Classes & Classifications Covered in this Patent:

Class 382: Image Analysis

This is the generic class for apparatus and corresponding methods for the automated analysis of an image or recognition of a pattern. Included herein are systems that transform an image for the purpose of (a) enhancing its visual quality prior to recognition, (b) locating and registering the image relative to a sensor or stored prototype, or reducing the amount of image data by discarding irrelevant data, and (c) measuring significant characteristics of the image.

Subclass 145: Inspection of semiconductor device or printed circuit board
Subclass 270: Variable threshold, gain, or slice level
Subclass 318: Multiple scanning