Study on the use of RET for improvement of dof in sub-via contact holes to severe topography

This paper investigates the application of resolution enhancement techniques for 140nm back-end via2 contact hole process with KrF system. On bare wafer, 120nm DoF (depth of focus) is secured for via2 process. However, different focal plane are formed due to the severe topography between cell and other region. And DOF of isolated contact hole is less than dense pattern. These factors lowered the overall DOF from 120nm to only about 80nm and raise many problems. Therefore, enhancement of aerial image is needed to improve DOF. In this study, mask type, mask layout, OAI (Off axis illumination) and SRAF (sub resolution assist feature) are applied all together in developed sequence. Because the isolated pattern and dense pattern are coexisted in via2 process, illumination and layout of mask are considered all at once. By using source mask optimization (SMO), optimum illumination and proper SRAF rule are developed and optimized. Through these processes, we obtain improvement of process window in comparison with existing condition. Furthermore coincidence of best focus is assured in real image on real conditioned wafer.