Defects in amorphous silica induced with monochromatic VUV rays

Photo-induced structural changes in amorphous SiO₂ films were examined using undulator radiation which was a monochromatic light source in vacuum ultraviolet (VUV) region. Frequency decrease of the Si-O stretching vibration in infrared absorption spectrum, generation of E′ centers, and increase of chemical etching rate were observed in the irradiated amorphous SiO₂ with 14.1 or 17.8 eV rays. These excitation energies correspond to excitations into excited states of the self-trapped exciton in amorphous SiO₂. In contrast, no change was observed in amorphous SiO₂ when irradiated with 7.2-12.6 eV rays. Frequency decrease can be explained with transition from regular six membered ring (6 Si and 6 O in a loop) to planar three membered ring (3 Si and 3 O in a loop) in amorphous SiO₂ network. Planar three membered rings and E′ centers are localized in the region of 5-25 nm from the top surface where chemical etch rate is also enhanced. A discussion of the mechanisms leading to structural change is presented.