System for designing mask pattern

A design system for designing a mask pattern for a given main pattern on a mask, wherein the given main pattern has an assistant pattern arrangement region around the given main pattern, the design system comprising:

• a defining module for defining a complementary pattern of the given main pattern by using a computer, wherein the complementary pattern is a complement of a reverse-tone pattern of the given main pattern;
• an adjustment module for shrinking the complementary pattern by using the computer to be a first shrunken pattern with a first shrinking proportion and to be a second shrunken pattern with a second shrinking proportion, wherein both of a first margin of the first shrunken pattern and a second margin of the second shrunken pattern are placed within the assistant pattern arrangement region and the first shrinking proportion is larger than the second shrinking proportion;
• a merging module for merging the first shrunken pattern and the second shrunken pattern by using the computer to define a reverse assistant pattern and further merging the reverse assistant pattern and the reverse-tone pattern to obtain a reverse pattern, wherein the first margin is an outer periphery of the reverse assistant pattern and the second margin is an inner periphery of the reverse assistant pattern;
• and
• a reverse module for reversing the reverse pattern to be a mask pattern by using the computer.