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Point defects strongly influence optical properties of synthetic amorphous silica (synthetic a-SiO2) used in excimer
laser photolithography and their properties are intensively studied. Decomposition of an Si-O-Si bond into a
pair of oxygen vacancy and interstitial oxygen species is an intrinsic defect process in a-SiO2. It is similar to the
creation of vacancy-interstitial pairs in crystalline materials and is regarded as "Frenkel defect process" in an
amorphous material. Oxygens are also known to be emitted from a-SiO2 surfaces under irradiation with vacuumultraviolet
(VUV) light or electron beam. However, the anion part of the Frenkel pair in a-SiO2, interstitial
oxygen atom, lacks reliable spectroscopic signatures. Therefore, Frenkel process has been studied much less than
another intrinsic defect process in a-SiO2, a simple cleavage of an Si-O bond, yielding a pair of silicon and oxygen
dangling bonds. Interstitial oxygen molecule (O2), a common form of the interstitial oxygen species in a-SiO2,
exhibits characteristic infrared photoluminescence (PL) at 1272 nm. This PL band allows interstitial O2 to be
detected selectively with a high sensitivity, and is useful in studying Frenkel defect processes in both a-SiO2
and crystalline SiO2. The Frenkel process is dominant over the formation of the dangling bond pairs in highpurity
synthetic a-SiO2. Both these processes are influenced by the degree of the structural disorder of a-SiO2characterized by distribution of Si-O-Si angles. Fluorine doping promotes the structural relaxation and is useful
in decreasing the concentration of "strained" Si-O-Si bonds, which have Si-O-Si bond angles widely different
from the relaxed angle and are vulnerable to radiation. Moderate fluorine doping is effective in improving both
UV-VUV transparency and radiation hardness, whereas heavy fluorine doping tends to enhance defect processes
involving the Frenkel mechanism and to degrade the radiation hardness.
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