Spectrophotometric determination of absorption in the DUV/VUV spectral range for MgF2 and LaF3 thin films

The intrinsic absorption of fluoride coating materials tends to higher values in the DUV/VUV spectral range. In respect to applications of this material class for DUV/VUV multilayer systems, like high reflecting mirrors and antireflective coatings, the control of absorption is essential for further improvement of the coating quality. However, the reliable determination of absorption losses by photometric techniques has to overcome various obstacles caused by light absorption in air, scattering effects of the coating, and absorption related to contamination of the employed fluoride material. In this contribution spectrophotometric measurements for the characterisation of optical data of MgF2 and LaF3 single layers and multilayer systems are presented. An advanced VUV/DUV-spectrophotometer, which has been developed at the Laser Zentrum Hannover for the optimisation of thin film production processes, is described. The subsequent optical data evaluation for fluoride single layer coatings is reported and the optical data, n and k, of MgF2 and LaF3 for the spectral range from 130 to 600 nm are presented. Transmittance and reflectance measurements of multilayer coating systems are compared to theoretical calculations for these systems on the basis of the evaluated optical data of the single layers. Observed deviations are discussed under respect of contaminations of the fluoride layer systems.