A completely new concept for designing the illumination aperture filter is suggested. From experimental or simulative methods, we have extracted the performance of every individual beam component on the illumination plane. The optimal apertures are then obtained by superimposing the best components that meet the requirements demanded by the specific photo process. Different kinds of optimal apertures were successfully implanted to deal with different process problems. Therefore, it is called the customized illumination aperture filter (CIF). The zero 1D OPE CIF, as a proof of concept, was designed to eliminate the OPE of low k1 process. Without any OPC, 0.6micrometers DOF of the common ED window was obtained, where k1 equals 0.39 for our NA equals 0.55 stepper to print. 0.18 micrometers line patterns, T push to smaller k1, another CIF was designed to maximize the individual DOF and overcome the reduced power problem accompanied with the typical aggressive OAI. Using this CIF, we achieved 1.1 micrometers common DOF with 7 percent EL for 0.18 micrometers lien patterns. The CIF doubles the power of the Nikon's strong quadrupole, shrine. An ultimate resolution limit of 0.11 micrometers line pattern was reached as well with the CIF. Finally, a contact/via CIF was designed combined with a halt-tone PSM. The CIF gives about 0.8 micrometers common DOF with 7 percent EL for 0.2 micrometers holes and 0.7 micrometers DOF for 0.17 holes using thinner resist. The CIF approach is, therefore, proven to be a cost effective and relatively easy realizable alternative to the alternating PSM for extremely low k1 process applications.