IR-ATR and IR transmission spectra of photo-CVD SiO₂ on Si have been measured to characterize the effect of F₂ treatment on the interface formation. IR-ATR spectra show that the Si-H bonds at the interface are reduced remarkably by the F₂ treatment. This reduction effect depends on the growth temperature, and the dependence is qualitatively similar to that of the interface states. IR transmission and XPS spectra show that the residual F in the SiO₂/Si structure is not bonded to Si. It is considered from these measurements that the F₂ treatment activates the Si surface to form tight Si-O bonds.