The process windows of L/S (equal line and space) features in critical dimension range of 0.25 to 0.20 micrometers with various illumination apertures are theoretically and experimentally studied in this paper. The purpose of this study is to evaluate the process capability of current state-of-the-art deep ultra-violet KrF stepper for 0.18 micrometers patterning by using off-axis illumination (OAI) technology. It is experimentally found that, by using OAI techniques, 0.2 micrometers L/S patterning can be achieved with enough depth-of-focus (DOF) if dummy lines are put on the edges of main patterns. Without OAI techniques (all lines should clearly be resolved), it is very difficult to achieve production worthy processes for 0.20 micrometers patterning because the image DOF had been dropped to around 1.0 micrometers . Comparing the performance among these apertures, the annular 2/3 aperture with NA equals 0.50 and (sigma) equals 0.8 is slightly better than annular 1/2 aperture with NA equals 0.55 and (sigma) equals 0.8 for 0.24 micrometers L/S feature patterning, while improvement becomes large for that of 0.20 micrometers L/S feature patterning. In the case of energy latitude, the energy latitude is about plus or minus 2 mj/cm2 for every aperture and feature, and the discrepancy among these apertures is very small. In simulation work, process parameters such as resist, development and optical systems were calibrated based on the results of conventional aperture obtained by experimental works. Prediction of experimental works can be achieved to a comfortable level, except for the case of annular 2/3 with NA equals 0.50 and (sigma) equals 0.8.