Abstract

Film of HfO₂ and mixed HfO₂/Al₂O₃ have been deposited on silica substrates for the purpose of obtaining low absorbing film for applications in excimer laser optics. Different samples were prepared by e-beam thermal evaporation with and without assistance of Al and Xe ions in oxidizing atmosphere. In Tab.1 the preparation methods are reported. All the substrates, Suprasil type 1, were cleaned in the same way with a last rinse in Acetone and dried with Nitrogen puff. The substrate surfaces showed no detectable absorption before the deposition. Substrate preparation and film deposition were performed in clean room (class 100). To achieve a resolution in the measurement of the optical absorption better than 20 ppm, a calorimetric technique, namely the photothermal deflection spectroscopy (PDS), was employed. To enhance the instrument detectivity the PDS measurements are usually performed in CCl₄, a liquid with a high temperature coefficient of the refractive index dn/dT. As the CCl₄ does not show a suitable transparency in the UV, alternative deflecting liquids were tested. We found Iso-Octane and Ethylene as the best candidates. When high purity Iso-Octane was used, a residual signal due to fluid absorption (the noise level) as low as 15 ppm at 250 nm was obtained. The scheme of the experimental set up is reported in Fig. 1.

© 1995 Optical Society of America