For the aim of fabricating next-generation semiconductor devices, researchers are now attempting to enhance 157-nm lithography so as to achieve 70-nm node level various circuit designs. Many of the challenges for 157-nm technology such as contamination and purge control, calcium fluoride intrinsic birefringence, finding resists with suitable performance characteristics, have been performed. The major challenge, in terms of stability of tool performance, has been the apparent accumulation of contamination on the bottom of the objective. This has been evidenced by a reduction in resolution performance and an increase in the non-uniformity of the illumination intensity across the image plane. Uniformity over the entire imaging field has increased from 0.58% to as much as 18.5% through the use of the tool. This paper reports our demonstration that loss of uniformity due to contamination from resist outgassing can be reversed by cleaning the bottom surface of the lens of 157-nm microstepper (Ultratech Stepper Inc.) in- situ using 157-nm light and a small concentration of O2 in the N2 purging for exposure area. With an in-situ oxygen (O2) and vacuum ultra violet (VUV) cleaning, the uniformity of over the full imaging field has been improved from 18.5% to 6.0%. The edges of the imaging field do not recover as well during a cleaning as the center of the field, as the central 0.5 mm diameter of the field uniformity has been improved to more or less 2.0%. The procedure of this in-situ O2 cleaning will also be introduced, and in addition to this in-situ O2 cleaning, some recent results in system performance will be shown and many of these challenges will be discussed.