The k1 factor continues to be driven downwards; the Extreme Ultra Violet Lithography (EUVL) should be a powerful solution for 2xnm node. But, EUVL is not ready for 2xnm node manufacturing currently. Therefore, we must extend ArF immersion capability on 2xnm devices. In order to enable the features/patterns of 20nm node and beyond, Mask Error Enhancement Factor (MEEF) and Depth of Focus (DoF) play an important role for continuing shrinking designs in the low-k1 lithography. Lithography optimization by RET (Resolution Enhancement Techniques) application is essential to obtain a usable process window (PW). SMO (Source Mask Optimization)  is a RET solution for better total process window improvement on 20nm node and beyond. Using these concern patterns of design rule, the optimal source with optical balance would be generated. The wafer result by using the optimal source need to be checked and compare with simulation result. In this paper, we will introduce how to use SMO in Cont lithography process development on 20nm node. The SMO of pattern split with PTD (Positive Tone Develop), single exposure with PTD (Positive Tone Develop) and single exposure of NTD (Negative Tone Develop) had been studied. Pattern split with PTD can provide an enough process window. But, it suffers overlay control and process cost issue. Single exposure is a good solution to fix overlay control and process cost. But it suffers low process window. Hence, single exposure with PTD is another choice to improve the process window. Base on our study, NTD SMO has better performance (DoF: ~20 increase, MEEF: ~10 decrease) than PTD SMO on single exposure process. The detail result will be shown in this paper.