A study of pupil filters for depth of focus enhancement in printing contact holes

In this paper, we investigate depth of focus (DOF) improvement using radially symmetric pure phase pupil filters. Optimization of such filters starts by requiring their corresponding point spread function be hat in the axial direction. It is found that DOF for 0.18 mu m (1:5) contact holes using an optimized filter is about 1 mu m, while printing them without filters is hardly feasible, using a binary mask. However, this filter introduces 2/3 intensity loss and therefore affects the processing throughput. We also apply the optimized filter to the imaging of 0.15 mu m contact holes and 0.18 mu m lines and spaces. For 0.15 mu m contact holes, it still performs better than other cases, but normalized image log-slopes are low for all cases. From the simulation data of 0.18 mu m lines and spaces, pure phase filters may not be good candidates for improving their DOF. Other DOF enhancing techniques such as quadrupole illumination may be required.