Proceedings Article | 30 December 2008
Proc. SPIE. 7132, Laser-Induced Damage in Optical Materials: 2008
KEYWORDS: Lithography, Deep ultraviolet, Silica, Laser induced damage, Annealing, Hydrogen, Laser irradiation, Excimer lasers, Temperature metrology, Absorption
We report on two approaches to strongly shorten life time testing of fused silica's absoption degradation upon 193 nm
laser irradiation. Both approaches are based on enhancing the two photon absorption (TPA) induced generation of E' and
NBOH defects centers in fused silica compared to common marathon test irradiation parameters. For the first approach
the irradiation fluence is increased from typical values H<1 mJ/cm<sup>2</sup> to H=10 mJ/cm<sup>2</sup>, therefore increasing the peak laser
power for a more efficient TPA process. To avoid microchannel formation in the samples, being a common break-down
criterion in marathon tests based on transmission measurements, a small sample of 10 mm length is irradiated and the
absorption is measured directly by the laser induced deflection (LID) technique. For comparing the experimental results
with a real marathon test at H=1.3 mJ/cm<sup>2</sup>, an experimental grade sample with very low hydrogen content, i.e. fast
absorption changes due to reduced defect annealing, is choosen. During the fluence dependent absorption measurements
after the prolonged irradiation at H=10 mJ/cm<sup>2</sup> it is found, that both experiments reveal very comparable absorption data
for H=1.3 mJ/cm<sup>2</sup>. For investigating standard material with high hydrogen content, i.e. slow absorption increase due to
effective defect annealing, a sample is cooled down to -180 °C in a special designed experimental setup and irradiated at
a laser fluence H=10 mJ/cm<sup>2</sup>. To control the increase of the defect density and to determine the end of the TPA induced
defect generation, the fluorescence at 650 nm of the generated NBOH centers is monitored. Before and after the low
temperature experiment, the absorption coefficient is measured directly by LID technique. By applying both, elevated
laser fluence and low temperature, the ArF laser induced generation of E' and NBOH centers in the investigated sample
is terminated after about 1.2*10<sup>7</sup> laser pulses. Therefore, a strong reduction of irradiation time is achieved in comparison
to about 10<sup>10</sup> pulses required in common marathon test applications.
