Optimizing integrated optical CD monitoring by floating pre-process variations in a complex multi-layer structure

Historically, in a volume production environment, process induced variation in optical property (n&k) of film stack was not significant for the most of applications using scatterometry. Many papers presented before addressed the CD variation in the production by adopting the fixed optical property approach [1-8]. However, with shrinkage of device size, and introduction of new material and process, n&k variation of some critical layers can not be ignored. In this paper, it presents impacts on measured optical CD due to n&k variation of one critical film in a 70nm DRAM ArF lithography process at a patterned area (A-layer). A solution to minimize the impacts using floating n&k in the scatterometry model is discussed, developed and verified.