Computer aided design software for designing phase-shifting masks

Computer aided design software has been developed in order to improve the efficiency of completing the design of Levenson-type phase-shifting masks. The software assists designers with the functions of automatic shifter arrangement and verification. In addition, to cope with contradictory spots where Levenson-type alternating phase shifting is impossible, the functions of extracting the shapes causing contradiction and of phase assignment with priority are also offered. The principle of shifter arrangement is that when the distance between clear mask areas is less than a certain threshold, a shifter is positioned into one of the clear areas. According to the principle, the number of pairs whose mutual phases have to be opposite decreases, and thus the degree of freedom to arrange shifters increases. The software was developed on a workstation, and its performance was evaluated using several layouts of 64Mbit dynamic random access memory. The average processing time for layouts with 300–400 shapes was about 1 min.