Soft breakdown properties affected by photon energy during rapid thermal post-oxidation annealing (POA) of ultrathin gate oxide are investigated. Generally, breakdown can be classified into normal hard breakdown (HBD) and soft breakdown (SBD). It was found that the occurrence of HBD and SBD depends on the process and stress field. Samples with front and back sides illuminated by a tungsten-halogen lamp during rapid thermal POA were examined. We find that front side illuminated oxides show easier SBD than back side ones. These results indicate that the photon energy may not only enhance the strength of Si-O bonds to suppress the lateral propagation of breakdown spots, but also to localize defects to form a conductive path of the leakage current. The understanding of the photon effect in rapid thermal POA is useful for controlling the electrical properties of ultra-thin gate oxides.