We investigate the benefit of sub-resolution assist features (SRAF) in improving the performance of potential depth of focus (DOF)-limited logic-like layers in 0.55 numerical aperture (NA) lithography. Our study involves a systematic evaluation of generic lines and spaces through pitch, as well as larger clips using the current Tachyon source mask optimization tool in both horizontal and vertical orientations for a darkfield tantalum boron nitride (TaBN) mask. We assess the robustness of the SRAF solution with respect to global processes and local mask variations. The impact of the variations is quantified using key lithography metrics, such as DOF and overlapping process window and process variation bands. We conclude that the presence of SRAFs reduces the impact of process and mask variability, and the benefit is orientation-dependent.