Photo process defect reduction is one of the most important factors to improve the process stability and yield in sub-0.18um DUV process. In this paper, a new approach to minimize the Deep-UV (DUV) Chemically Amplified Resist (CAR) and Bottom Anti-Reflective Coating (BARC) induced defect known as UFO (UnidentiFied Object) defect will be introduced. These defects have mild surface topography difference on BARC; it only exists on the wide exposed area where there is no photoresist pattern. In this test, Nikon KrF Stepper & Scanner and TEL Clean track were used. Investigation was carried out on the defect formulation on both Acetal and ESCAP type of photoresist while elemental analysis was done by Atomic Force Microscope (AFM) & Auger Electron Spectroscopy (AES). Result indicated that both BARC and photoresist induce this UFO defect; total defect quantity is related with Post Exposure Bake (PEB) condition. Based on the elemental analysis and process-split test, we can conclude that this defect is caused by lack of acid amount and low diffusivity which is related to PAG (Photo Acid Generator) and TAG (Thermal Acid Generator) in KrF photoresist and BARC material. By optimizing photoresist bake condition, this UFO defect as well as other related defect such as Satellite defect could be eliminated.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.