248-nm lens materials: performance and durability issues in an industrial environment

Abstract

The short wavelengths and high peak powers of rare gas-halide excimer lasers suggest many uses in industry. To exploit these opportunities, lasers must be developed that can operate reliably and economically at high repetition rates and high pulse energies, and optics must be fabricated that can withstand hundreds of millions or billions of laser shots without degradation. On the laser front, rapid progress is being made by several manufacturers. This talk focuses on the optics, in particular, transmissive optical materials for KrF laser applications. A variety of synthetic fused silicas were tested in front of an industrial KrF laser at high repetition rate (300 Hz) and moderate fluence (500 mJ/cm2). All samples develop a characteristic absorption band at 210 nm in the early stages of the exposure. The absorption relaxes gradually after the laser is turned off. In almost every sample, this gradual behavior is followed by a large and sudden increase in the 248 nm absorption (up to 12% per cm). The nature, consequences, and possible causes of this `strong absorption transition' are discussed. Some data on high-purity single-crystal CaF2 is included for comparison.