High-performance drying techniques using IPA (isopropyl alcohol) are widely used in the silicon wafer cleaning process. IPA-based drying techniques help prevent the formation of watermarks because they effectively displace any water remaining on a wafer surface. They are thus frequently used in the single wafer cleaning system for advanced devices in which ultra-clean process performance is required. However, as devices are becoming physically smaller, the formation of extremely small defects during cleaning has become a serious problem. It is therefore important to elucidate the mechanism of the defect formation and to take measures to prevent it for future device technologies in which small-size defects can be killer defects during production. In this paper, we performed experiments focused on the process chamber atmosphere in IPA drying of the single wafer cleaning system and describe the mechanism of the defect formation.