Interaction of F2 excimer laser with SiO2 glasses: Towards the third generation of synthetic SiO2 glasses

Abstract

Changes in optical absorption spectra and defect formation of different types of synthetic SiO2 glasses were examined by irradiation with F2 excimer laser pulses (157 nm). Fluorine-doped, OH-free SiO2 glasses exhibit high optical transmittance (∼80%) at 157 nm in an as-delivered state and the intensity of F2-laser-induced absorption is much less than that in wet or dry F-free samples. The effect of F-doping on the blue shift of the absorption edge and suppression of color center formation was conspicuous up to 1 mol% but was slight upon further doping. It is suggested that elimination of strained Si–O–Si bonds upon F-doping is the primary reason of the improvement of resistance of SiO2 glasses to F2-laser light. Novel optical phenomena by F2-laser irradiation, bleaching of the vaccum UV (VUV) absorption edge and changes in the SiOH infrared absorption, were found in H2-impregnated, or wet SiO2 glasses. These results lead to the conclusion that F-doping to 1 mol% is an effective and practical method to obtain synthetic SiO2 glasses for F2 excimer laser optics as a photomask in optical lithography.