i-line wafer steppers are evolving as established production tools, and it is evident that they will be used to realize features in the sub-half-micron region. Consequently, i-line steppers can be expected to be the equipment of choice for volume production of 16 MBit DRAMs and possibly the first generation of 64 Mbit devices, before the introduction of DUV lithography. However, for this sub-half-micron resolution, lenses with higher apertures and large field sizes will be required. In this paper a new family of wafer steppers is introduced, with a new mechanical frame design and modular architecture which can accommodate a family of large field i-line and deep UV lenses. Results from the first lens type with NAequals0.54 and a field diameter of 25.5 mm are described. To overcome the anticipated depth of focus problems on production wafers, a field-by-field leveling system is introduced, ensuring optimum focus over the whole image field. A challenging problem of wafer steppers using this option is the alignment accuracy during stepping of stage and active leveling of the wafer chuck. The stepper concept introduced here is able to realize the field-by-field leveling without the need for the throughput consuming field-by-field alignment. For that purpose a wafer stage with a new metrology system and improved accuracy has been designed, resulting in an overlay accuracy better than 85 nm in the global alignment mode. Simultaneously a throughput of more than 80 150 mm wafers per hour is realized. Along with the new lens and metrology concept, the stepper contains local environmental control systems performing better than class 1, to ensure clean handling of 8-inch wafers without the need for space consuming environmental enclosures. This paper reports practical results from the new stepper, including resist features below 0.4 micrometers , overlay measurements, particle control, as well as a general description of the new stepper architecture.