This paper describes the interesting performance for KrF excimer laser lithography at low k1 process in Rayleigh criteria. For 0.15 micrometer lithography process, the experiments have been performed by using resolution enhancement techniques (RET) such as phase shift masks (PSM), off-axis illumination (OAI), 0.63 high NA, and high contrast resist. Especially for the gate level CD control, we have investigated the thin film interference effects of oxide, resist and BARC thickness variation by simulation and experimental. Also, we have experimentally compared the CD control performance and the process window with oxide thickness variation on actual device wafers, because the oxide thickness variation enhances thin film interference effects. Finally, we determined the most suitable conditions for providing sufficient protection against reflection by controlling the substrate film thickness. Furthermore, requirements for 0.15 micrometer gate level CD control are discussed.