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13 May 1997 Pulse-length dependence of absorptance and degradation rate of fused silica at 248 nm
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Optical components for industrial excimer laser applications such as semiconductor lithography steppers have to withstand more than 1010 laser pulses without suffering from significant cumulative absorption increase due to color center formation. Lithography lasers are now approaching pulse durations of 10 ns and repetition rates of 1000 Hz, a domain unexplored in previous degradation studies. With the aid of an optical pulse-extension unit and a modified laser resonator, we measured absorptance and absorption changes in Suprasil 312 UV-grade fused silica calorimetrically at pulse lengths of 7-73 ns. At 100 mJ/cm2, no degradation was detectable even after 500,000 pulses, but there was slight evidence of an absorption decrease due to laser cleaning. At 400 mJ/cm2, both absorptance and degradation rate increased towards shorter pulses and hence higher intensities, but even over the full pulse-length range, the changes remained moderate. Furthermore, degradation at 300 Hz was lower than at 150 Hz, indicating that a simultaneous bleaching effect might be involved which favors future high- repetition rate applications.
© (1997) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Eric Eva, Klaus R. Mann, and Stephan Thomas "Pulse-length dependence of absorptance and degradation rate of fused silica at 248 nm", Proc. SPIE 2966, Laser-Induced Damage in Optical Materials: 1996, (13 May 1997);

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