Jens Ullmann

Application of the Tauc-Lorentz formulation to the interband absorption of optical coating materials

Recent progress in ellipsometry instrumentation permits precise measurement and characterization of optical coating materials in the deep-UV wavelength range. Dielectric coating materials exhibit their first electronic interband transition in this spectral range. The Tauc-Lorentz model is a powerful tool with which to parameterize interband absorption above the band edge. The application of this model for the parameterization of the optical absorption of TiO2, Ta2O5, HfO2, Al2O3, and LaF3 thin-film materials is described.

Coated optics for DUV excimer laser application

Recent developments of DUV-excimer laser applications have gained in demands for radiation resistant coated components at interesting wavelengths. To meet the requirements of long term reliability and high pulse number throughput superior performance of the optical components with lowest absorption and scattering losses are necessary. In the framework of the German Joint Research Project OPUS II efforts are made to investigate the optical properties, the radiation resistance and long term stability of single layers and layer system of interest in the DUV. The evaluation of optical coatings and coating system on different substrate materials was carried out by scattering experiments, atomic force microscopy, IR spectroscopy, calorimetric absorption measurements, and determination of laser induced damage threshold. Additionally, from the spectralphotometric measurements the optical behavior of the films was examined.

Mechanical stress in fluoride coatings

The mechanical behavior of fluoride single layers and thick model layer systems were carried out ex-situ after deposition at laboratory conditions. The samples on substrates with different thermal expansion coefficients have been prepared by a low loss evaporation process. Investigation were performed for single layers deposited at different film thickness, substrate temperature and storage time. All investigated fluorides posses tensile stress of various amounts. The thermal stress component seems to be the major contribution to the total stress MgF2 films of about 100 nm thickness deposited onto silicon substrate. The stress and force per unit width of model multilayer systems formed from different fluorides were examined and correlated to the ability of crack formation on thick rigid fused silica substrates. Optimization of the stress values, the bending force and the crack formation was performed by adjusting the deposition temperature and by introducing a third stress reducing material in the stack. An example of a stress optimized high reflector will be shown.

Structural and mechanical properties of evaporated pure and mixed MgF2-BaF2 thin films

To grow dense and hard MgF2 films substrate temperatures of about 300 degrees C are required, which unfortunately leads to high tensile film stress and the ability of crack formation. Lowering tensile stress in MgF2 films can be achieved by admixture a second fluoride material of higher cation radius than Mg2+. While former investigation were performed with non-heated films the purpose of the present work was to verify the behavior of mixed films when deposited at elevated substrate temperatures. One of the promising add material is BaF2 which enables evaporation of appropriate pre-mixed materials from a single source. The BaF2 content in the mixed films was varied from 3 to 55 mol percent in the MgF2 host. Optical, mechanical, and structural properties of samples deposited at different substrate temperatures have been studied by spectral photometry, IR spectroscopy, ex situ measurement of mechanical stress, x-ray diffraction, and -reflectometry, RBS, as well as investigation of surface morphology.