During plasma processes, the SiO2/Si interface state is generated by UV photon irradiation. We measured Pb centers (Si dangling bond at SiO2/Si interface) in SiO2/Si film by electron spin resonance spectroscopy. The density of Pb centers increased with increasing UV photon energy. This result indicates that high-energy photons effectively generate Pb centers because of the absorption at the SiO2/Si interface. At the same time, we found that UV photon flux is also an important factor in increasing the density of Pb centers. On the other hand, ion irradiations were not a significant factor in the generation of Pb centers. Ions can penetrate only the surface (less than 5 nm) but they cannot induce a Pb center at the SiO2/Si interface. Furthermore, pulse-time-modulated plasmas were used to reduce the UV photon irradiation during the plasma processes. The reduction in UV photon irradiation could completely eliminate the Pb centers compared with the use of a conventional continuous wave plasma.