To achieve advanced contact layer printing, there always are two key factors need to be handled: resolution and through-pitch common process window. Among all solutions, the most common approach is off-axis illumination (OAI) + attenuated phase-shift mask (att-PSM) + sub-resolution assistant features (SRAF). With adequate/high Numerical Aperture (NA) and OAI settings of the leading edge scanners, the resolution should not be a problem, while even with att-PSM + SRAF, the through-pitch common photo process window still leaves much to be desired. This phenomenon is due to the existence of forbidden pitch - under certain illumination condition, there always exists a pitch range which has no spacing for insertion of SRAF while contrast is still poor and needs some special treatment to enhance the image qualities. This invention and study is to use special SRAF, we call DAF (Diagonal sub-resolution Assistant Feature), to enhance the process performance of forbidden pitches. The main methodology is to select the so-called "forbidden-pitch" structures from the whole database, then apply our DAF rules. After that, apply Conventional sub-resolution Assistant Feature (CAF) rules on post-DAF full-chip database, finally comes OPC treatment. With this approach, we demonstrate excellent results on 65nm contact layer, showing no forbidden pitch and sufficient large through-pitch photo common process window via simple OAI (ArF, 0.82NA, 1/2Ann.) + att-PSM + SRAF.