Inverse mask synthesis, or Inverse Lithography Technology, as a next generation resolution enhancement technology,
is drawing pretty much attention after years of development. However, the existing optimized mask usually
is too complex such that the pattern simplifying procedures have to be applied as a post processing step. But
the post processing step may lead to pattern degradation and unwanted side lobe printing. In this paper, we
first implement a new inverse mask synthesis system using two dimensional discrete cosine transform(DCT2)
of the target mask, where the low frequency components are used in the optimization. As the high frequency
components are discarded, the resulted optimal pattern is similar in shape to that of using the level set method
in the published papers. Moreover, as inverse mask synthesis is an ill-posed problem, there are some local minimum
locations. Previous algorithms usually use the desired pattern as an initial iteration point, in the sense
that optimized pattern shall be a perturbation of the desired pattern. A common fact is that initial solution
is critical to the optimization procedure and final result. In this paper, we apply an initial SRAF insertion
around the main features before starting the existing inverse engine. The SRAF insertion does not need to be
as accurate as that in the traditional SRAF+main feature OPC flow. Therefore, it does not add higher time
burden on the whole mask synthesis flow. We implement the SRAF insertion based on computed mask electric
field distribution. The experimental results show that using the initial fast SRAF insertion, the inverse engine
is able to take advantage of a better initial high contrast image distribution, and the optimized pattern can be
much simpler while the pattern fidelity is still in good control. We also observe that better optimized patterns
can be achieved with fewer iterations.
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