Benefits of using advanced sub-resolution features for 0.55NA brightfield imaging

“Low-n” bright field masks are key to enabling single exposure in 0.55NA at lower dose and improved contrast. However, these masks come with best focus shifts (BFS) for isolated features and an increased risk of assist feature (AF) printing. In this paper we describe how segmentation of AFs can minimize their printing risk without violating mask manufacturing limits for simple line spaces. They can also be used to address BFS and increase the depth of focus. The trade-off between AF print margin and the DOF performance is also considered. Finally, the impact of assist feature in reducing sensitivity to process variations is demonstrated on a logic clip.