A novel illumination system for mask aligners will be described. The illumination system provides improved exposure light uniformity and customized illumination. It is based on two consecutive Kohler integrators and an exchangeable illumination filter plate. It allows a free choice of illumination settings, e. g., ring-illumination, quadrupole, multipole, Maltese cross, and free-forms in a standard mask aligner. The so-called micro optical exposure optics significantly increases the field of applications for mask aligners. The well defined illumination allows optical proximity correction (OPC) of the mask pattern to compensate for the remaining image errors due to diffraction or process effects. Customized illumination and OPC-like structures introduce well-known tools of projection lithography for mask aligners for the first time. Lithography process simulations as well as experimental results will be shown. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3518920]