Reduction in the illumination wavelength for exposure leads to higher resolution while keeping the depth of focus. Thus, KrF excimer laser lithography has been positioned as the next generation lithography tool behind g/i-line optical lithography, and many studies have been investigated. In the early days, the excimer laser lithography had many inherent problems, such as inadequate reliability, difficult maintainability, high operating cost, and low resolution and sensitivity of resist materials. However, the performance of the excimer laser stepper has been improved and chemical amplification resists have been developed for the past decade. At present, KrF excimer lithography has reached the level of trial manufacturing of lower submicron ULSI devices beyond 64 Mbit DRAMs.