Long-term effects of pulsed KrF laser radiation on crystalline and amorphous sio₂

Abstract

At the 1992 Boulder Damage Symposium, we reported test results for 6 fused silica glass types exposed to millions of KrF laser pulses at a fluence of 500 mJ/cm2 and a pulse repetition rate of 300 Hz. The peculiar results and the big variations between glass types ("slow" relaxing vs. "fast" relaxing glass families) were interpreted as being due to subtle differences in the glass micro structure induced by the homogenization and heat treatments, but no detailed explanation could be offered. As a small step toward improving our understanding, we have expanded our studies to include four additional glass types: Suprasil 300 (S300) and Suprasil 311 (S311) from Heraeus, and conventional 7940 and a new Excimer-grade fused silica from Corning. For comparison, we have run samples of high-purity cultured α-quartz from Sawyer Research Products using the same sample geometry and irradiation conditions. The quartz crystals develop minor UV absorption at 210 nm, with negligible absorption at the 248 nm laser wavelength. The two Corning glass types behave similarly to Suprasil 2 and 3 under our irradiation conditions, and thus belong to the slow-relaxing family. S300, the only "dry" fused silica included in our study, performs miserably under our test conditions, confirming literature and anecdotal evidence that dry fused silicas have much poorer UV resistance than the wet varieties. The behavior of S311 is qualitatively different from any of the other "wet" fused silicas that we have tested. The UV absorption increased rapidly at the beginning of the test but reached its worst-case condition in just 20 minutes of exposure time. Thereafter the absorption decreased gradually during a 72 million shot exposure. S311 exibits negligible spontaneous relaxation of the UV absorption when the laser is halted, unlike the other glass types which show slow to fast relaxation of the UV absorption if the laser is halted before the strong absorption transition.