In recent year, the requirements of semiconductor packaging technology have become smaller and finer, while the miniaturization of the semiconductor and the shrinkage of the package size are demanded. The fine process as used semiconductor manufacturing process is tried to apply as production for semiconductor packaging process, but it is so difficult due to large size work, different material, controlling process conditions and so on. We have been studying the practical application of the Photodesmear® process using vacuum ultraviolet (VUV) light 1-10). Photodesmear® processes that utilize high power VUV light and high oxygen concentration atmospheres have the potential to replace desmear processing in future semiconductor packaging processes. In this study, we particularly focused on the oxygen flow velocity in optimizing the Photodesmear® process. We evaluated the uniformity of the desmear process with a large practical size device and found the problems associated with the increase in size. By measuring and calculating the ozone concentration and oxygen concentration in the atmosphere, the factors of the flow velocity dependence of the reaction gas were considered for the improvement of the uniformity in the Photodesmear® process.