In 65nm and beyond generations, contact/via patterning is more challenging due to the complexity of manufacturing masks and the weak lithography process window. High NA scanners and suitable illumination can provide the desired resolution and dense pitch. However, there are trade-offs between process window, mask error enhancement factor (MEEF), and proximity effect. Some assistant technology is reported in literature, such as thermal flow, RELACS, SAFIER and sub-resolution assistant features. In this paper, we report a detailed study of the feasibility and limitations of these kinds of methods. Finally, we describe sub-resolution assistant features when used in QUASAR illumination with lower sigma, which have shown great promise to reduce the proximity effect and MEEF to get a larger lithography process window.