Sub-resolution assist feature (SRAF) study for active area immersion lithography

Sub-resolution assistant feature (SRAF) is applied to enhance the process window of isolated and semi-isolated features by taking advantage of the optical interference between the main features and the assistant features. SRAF is an essential technique for advanced immersion lithography. Advanced node requires both tight critical dimension (CD) control and enough process window for critical layer like the active area (AA) layer. Scattering bar (SB) is used to improve line pattern process window while reverse scattering bar (RSB) is used to improve trench pattern process window. Both SB and RSB are used for the AA layer. Therefore, it is challenging to balance the lithographic process window performance indicators, such as, exposure latitude (EL), depth of focus (DOF), and the SB and RSB printability. This paper will discuss AA layer´s SRAF size, SRAF placement for different pitches in advanced immersion lithography. Furthermore, finite-difference time-domain (FD-TD) simulation study was done to accelerate SRAF rule selection for both SB and RSB. Wafer level data were also collected to confirm process window and SB/RSB printability issue.