Paul M. Schermerhorn

Deep‐ultraviolet damage to fused silica

Several experiments using 213‐nm radiation have been performed with the goals of characterizing and understanding better the mechanisms behind UV damage to fused silica. A novel method of monitoring compaction in real time was discovered which measures the amount which the damaged fused silica depolarizes the incident beam. Refractive index changes of less than one part in ten million were resolved. Compaction induced stress was observed to extend well beyond the irradiation site with a maximum at the edge of the irradiation site. Detection of compaction occurred much earlier than the detection of color center absorption when fused silica was irradiated. The rates of both compaction and of color center formation depend superlinearly on pulse energy density and can be fitted by a quadratic function, suggesting a two‐photon damage process. In the same energy density range, the absorption coefficient depends linearly on energy density, from which we extract a 213‐nm two‐photon absorption coefficient of 5×10−...

Excimer laser damage testing of optical materials

Photolithography technology is moving towards deep UV wavelengths in order to obtain smaller feature sizes on integrated circuits. Optical glasses are being replaced with fused silica because of the DUV application. There are concerns that the short pulse, high instantaneous power generated with excimer lasers can cause structural changes in the fused silica materials. The damage includes: changing absorption, observed fluorescence, and induced stress birefringence. This paper will address the laser damage testing protocol for fused silica. We will discuss the setup, equipment, and procedures for laser damaging. Precision measurements of the absorption, fluorescence, and stress birefringence are required to have a good understanding of the laser damage. The paper will also address the equipment problems associated with these measurements and initial comparisons between 193 nm and 248 nm laser damage.

Ultraviolet damage properties of various fused silica materials

Five different fused silica types were evaluated for their resistance to UV-induced compaction and color center formation at 193-nm. Real-time monitoring of color- center-induced absorption showed three distinct dependencies of transmission on pulse count. The initial rates of color center formation varied by well over a factor of ten between the materials tested while compaction-induced birefringence rates varied by at most a factor of four. Of the likely candidates for lithographic applications, Corning Excimer Grade 7940 fused silica was the least prone to color center formation while Suprasil 311 showed the lowest compaction rates. The rates of compaction-induced birefringence and color-center-induced absorption from 213-nm radiation were found to increase dramatically under elevated sample temperature conditions. Since a two- photon absorption mechanism is believed to be the catalyst for UV damage to fused silica, two-photon absorption coefficients were characterized at elevated temperatures. The two-photon coefficients at 213-nm for all materials measured including crystalline quartz and under all applied conditions were statistically equivalent, leading to the conclusion that the energy dissipation mechanism, in addition to two-photon absorption, is important to UV damage to fused silica.

Excimer laser induced absorption in fused silica

Excimer laser radiation changes the physical and optical properties of fused silica. These changes include compaction of the glass and induced absorption, both of which have an impact on the expected lifetime of silica lenses used in optical microlithography. We report on our ongoing study of excimer laser induced changes in fused silica. We use a fully automated experimental setup designed for marathon exposure of the sample at low fluence. In each setup, using either an ArF or a KrF laser, up to five samples are exposed simultaneously and their induced absorption is measured in situ. The spatial and temporal profiles of the laser beam can also be measured in the same setup. We present and discuss results from marathon test of fused silica at fluences close to the conditions expected in optical microlithography systems.