A highly reliable mask inspection system

An automatic system for inspecting micro mask defects with 1-µm minimum detectable size has been developed. An outline of the system is as follows: The pattern image obtained with a pickup tube is converted into binary video signals which are transferred into two parallel logic circuits for detecting pattern defects. One is based on the pattern-analyzing method, for which one of four algorithms for detecting micro defects is presented in detail. The other is based on the design-pattern data-comparing method, where the data compression scheme and a new idea for avoiding mask alignment errors are adopted. A software system outline, very important in assisting the hardware functions in this system, is also presented. The results of experiments for determining system performance indicate that the system can detect ≥1-µm diameter defects or loss patterns with high probability by complimentary use of the two methods. A 4-in by 4-in mask can be inspected within 100 rain.