A. Field of the Invention
This invention relates generally to object inspection apparatus, and relates more particularly to an apparatus and a method for inspecting photomasks having duplicate die patterns thereupon for defects by utilizing an area subtraction technique to identify defects as differences between otherwise duplicate die patterns.
B. Description of the Prior Art
Automatic photomask inspection systems have been commercially available for a number of years. Two such systems are shown in U.S. Pat. No. 4,247,203 issued Jan. 27, 1981 to K. Levy and P. Sandland and entitled "Automatic Photomask Inspection System and Apparatus," and U.S. Pat. No. 4,347,001 issued Aug. 31, 1982 to K. Levy and P. Sandland and also entitled "Automatic Photomask Inspection Systems and Apparatus." The machines disclosed in these patents found defects by comparing digitized images of two adjacent die patterns and recognizing differences between the two as defects. One drawback to the photomask inspection systems disclosed in the Levy patents is that the efficiency of the defect detection circuitry was poor for defects located near corners and on diagonal lines.
Photomask inspection systems that form pixel images of a photomask to be inspected are known in the prior art. For example, the Levy patents, listed above, disclose a system that optically projected an image of two die patterns onto photosensitive sensors. The sensors were scanned periodically as the photomask moved to form a two-dimensional pixel representation of the die patterns. These pixel representations were stored in digital form and processed by defect detection circuitry to locate and identify defects.
One method of processing such two-dimensional pixel representations of die patterns would be to first formulate and store both pixel representations in their entirety in a memory, then to shift the pixel representations to correct for alignment errors, and then to perform a comparison between the two pixel representations to identify regions where they do not match. A significant drawback to this method is the huge memory that would be required for the billions of bits of information necessary to represent die patterns with sufficient precision for state-of-the-art integrated circuits.
Another method of processing two-dimensional pixel representations of die patterns to determine defects would use a computer to process the sensor outputs as they are generated by the sensors. While this method would substantially reduce the memory requirements as compared to the method mentioned immediately above, the execution time of the programmed instructions necessary to process the sensor data would be prohibitively lengthy.