As the pattern size scales down and the wafer size increases, the particle defects should be controlled more strictly in the modern microelectronics fabrication. Single wafer cleaning processes are introduced in the industry due to higher particle removal efficiency and lower particle reattachment possibility. In this study, the origins of watermark defects in single wafer cleaning system were identified, and the ways to avoid the problems were sugested through a model based numerical simulation. The watermark defects observed in the single wafer cleaning systems were found to originate from the failure of controlling water and iso-propyl alcohol (IPA) relative humidities in the surrounding gas. Increasing the IPA relative humidity in the surrounding gas during the drying phase could improve watermark defects and increase product yields. (c) 2007 Elsevier Ltd. All rights reserved.