Sub-Resolution Assist Features Challenge and Solution in 28nm Active Area Lithography

Sub-resolution assist features (SRAFs) provide an absolutely essential technique for critical dimension (CD) control and process window enhancement in immersion lithography. The selection of SRAF’s size and placement are vital. 28nm node requires stringent CD control for active area (AA) layer. Since both AA line CD and space CD control are important, not only scattering bar (SB) but also reverse scattering bar (RSB) is used to improve AA litho process window. SB is used to improve line pattern process window, meanwhile RSB is implemented to improve space pattern process window as space CD shrinks as small as possible to meet design rule requirement. The main challenges of SRAFs are the proper SRAF size, right SRAFs placement, and avoiding SRAFs printing issue. SRAF size needs to balance process window and SRAF printing issue. Further more, mask making feasibility is also considered. 28nm AA lithography SRAFs rule simulation study with FDTD is used to optimize the AA litho process. 28nm AA test vehicle with SB and RSB size split and location split is designed to determine the integrated SRAFs solution. After implement thorough considered SRAFs rule, 28nm AA litho through pitch line and through pitch space process window can meet manufacturing specification.