Comparative study of fused silica materials for ArF laser applications

Abstract

We report on a comparative study of a variety of fused silica materials for ArF laser applications, which differ regarding their OH content. Laser induced deflection (LID) technique is applied to measure directly and absolutely the absorption coefficient in fused silica materials at 193 nm as a function of the incident laser fluence in the range 1...3 mJ/cm² before and after applying 20 million shots at a fluence of 5 mJ/cm². In addition the laser induced refractive index change is detected by interferometer measurements after the prolonged irradiation for all samples. Prior to the long term irradiation, low OH containing fused silica ([OH] < 80 wt-ppm) exhibits both, the lowest absorption coefficient and the lowest absorption increase with fluence (dk/dH) in the range 1...3 mJ/cm². During 20 million laser pulses at 5 mJ/cm², however, the absorption and the dk/dH values show a strong increase for the low OH containing fused silica. In contrast, the absorption of the medium OH containing samples ([OH] = 200...650 wt-ppm) is highest prior to the long term irradiation but is remarkably lowered throughout the 20 million laser pulses. High OH containing fused silica ([OH] > 900 wt-ppm) shows an intermediate absorption level, which only slightly increases or decreases during the irradiation with 20 million laser pulses. The ArF irradiation induced refractive index change is positive (= compaction) for all samples at the fluence 5 mJ/cm². For analysis, a particular material classification is taking into account. For low and medium OH containing samples, referred to as compaction-dominated, the compaction factor increases with the OH content. For high OH containing samples, referred to as rarefaction-dominated, the resulting compaction factor decreases with increasing H₂ content.