Cluster size effects of SiO2 thin film formation with size-selected O-2 gas cluster ion beams (GCIBs) irradiation on Si surface were studied. The cluster size varied between 500 and 20,000 molecules/cluster. With acceleration voltage of 5 kV, the SiO2 thickness was close to the native oxide thickness by irradiation of (O-2)(20,000) (0.25 eV/molecule), or (O-2)(10,000) (0.5 eV/molecule). However, it increased suddenly above 1 eV/molecule (5000 molecules/cluster), and increased monotonically up to 10 eV/molecule (500 molecules/cluster). The SiO2 thickness with 1 and 10 eV/molecule O-2-GCIB were 2.1 and 5.0 nm, respectively. When the acceleration voltage was 30 kV, the SiO2 thickness has a peak around 10 eV/molecule (3000 molecules/cluster), and it decreased gradually with increasing the energy/molecule. At high energy/molecule, physical sputtering effect became more dominant process than oxide formation. These results suggest that SiO2 thin film formation can be controlled by energy per molecule. (C) 2009 Elsevier B.V. All rights reserved.