TESTING OF THE DURABILITY OF SINGLE-CRYSTAL CALCIUM-FLUORIDE WITH AND WITHOUT ANTIREFLECTION COATINGS FOR USE WITH HIGH-POWER KRF EXCIMER LASERS

The transmission of uncoated and antireflection (AR)-coated CaF2 windows has been measured during long KrF laser exposures. Samples were tested for up to 75 million pulses in front of an industrial-grade KrF excimer laser at high repetition rates (200-300 Hz) and moderately high energy fluence (220-550 mJ/cm2). In this fluence range bare CaF2 performs much better than the best fused silicas currently available. Various AR coatings (whose structures were initially unknown to us) were deposited on CaF2 substrates polished to two distinct surface finishes. We determined the coating structures by using x-ray fluorescence spectroscopy to identify positively all elements that were heavier than fluorine and Rutherford backscattering spectroscopy to determine the layer structure and approximate layer thicknesses. We confirmed the structures by comparing measured UV transmission spectra with computer simulations. This study reveals excellent performance for SiO2/Al2O3/SiO2/Al2O3 coatings on highly polished substrates, which provided two-sided transmission in excess of 99.8% at 248 nm and no measurable deterioration after 75 million pulses. A MgF2/Al2O3 coating also performed quite well, while a hafnia-containing coating (MgF2/HfO2/FaF3) exhibited anomalous behavior. All-fluoride coatings consisting of MgF2/LaF3/MgF2 and MgF2/LaF3/AlF3 had good durability but lower starting transmission as a result of scatter losses in the LaF3 layer. These results demonstrate the availability of AR-coated CaF2 optics for high-power KrF laser applications with high transmission and no sign of degradation up to approximately 7 x 10(7) pulses.