Source Optimization Using Simulated Annealing Algorithm

As lithography still pushing toward to lower k(1) imaging, traditional illumination source shapes may perform marginally in resolving complex layouts, freeform source shapes are expected to achieve better image quality. Illumination optimization as one of inverse lithography techniques attempts to synthesize the input source which leads to the desired output wafer pattern by inverting the forward model from mask to wafer. This paper proposes a method to optimize illumination by using simulated annealing algorithms (SA). A synthesis of the NILS values at multi-critical mask locations over a focus range is chose as the merit function. The advantage of the SA algorithm is that it can identify optimum source solutions without any additional apriori knowledge about lithographic processes. The results show that our method can provide great improvements in both image quality and DOF.