Induced absorption in silica (a preliminary model)

The use of silica lenses in a photolithographic system employing a 193nm excimer laser has been proposed. It is desirable to determine if, at the low intensity to be used in the system (approximate to 0.1 mJ/cm(2)), the glass will withstand about ten years of use without objectionable induced absorption. At a pulse frequency of 1Khz, this length of time corresponds to about 10(11) pulses. Because of the long time involved, an accelerated test is needed to determine the susceptibility of silica to induced absorption. The mechanism of darkening must be understood in order that the behavior of the glass under use conditions be predicted with confidence from the results of the accelerated test. The most important processes in the mechanism of induced absorption are: 1) Two photon absorption creating an exciton 2) Trapping of the exciton by a localized state 3) Dissociation of the trapped exciton to form an E' center and a NBOHC 4) Reaction of these centers with hydrogen to form SiH and SIGH bonds. 5) Photolysis of the SiH bonds to produce more E' centers. The mechanism will be discussed in detail and the agreement with experimental results over a range of intensities and hydrogen levels will be presented.