Challenges and solutions of few-line patterns with freeform illumination in immersion lithography
Lithography is facing great challenges from the continuous shrinkage of critical dimension in order to satisfy the development of the technology node. To extend ArF immersion lithography to sub 20 nm technology node, it is necessary to print smaller pitch size such as 80 nm for 1D pattern with enough process window. But it is hard to compromise 2D and other more complicated features. Source-mask optimization (SMO) is one of the key techniques for sub 20 nm technology node, which combines freeform light source with mask optimization to achieve a good process window for a variety of key features which optical proximity correction has limited solutions. In this paper, we use SMO technique for different kinds of pattern which is typical in sub 20 nm node, including 1D pattern, 2D regular pattern such as line-end to line-end, and 2D irregular pattern. It is shown that a proper source with a proper mask optimization strategy can be obtained, which gives enough common process window on most of the test pattern. However, few-line pattern like 3-line, 4-line or 5-line which have some special pitch and are common in real chip, are very hard to print with enough process window even with SMO technique. In this paper, we present several solutions such as pattern shift and broaden for few-line pattern. The process window can be improved with these solutions.