Comparative study of the UV-optical and structural properties of SiO2, Al2O3, and HfO2 single layers deposited by reactive evaporation, ion assisted deposition and plasma-ion-assisted deposition

Growing requirements on the optical and enviromnental stability as well as on the radiation resistance against high power laser radiation especially for optical interference coatings used in the ultraviolet spectral range have to be met by new optimized thin film deposition technologies. For applications in the UV spectral range the number of useful oxide thin film materials is very limited due to the higher absorption at wavelengths near to the electronic band gap of the materials. Applying ion assisted processes offer the ability to grow dense and stable films, but in each case careful optimization of the eposition process (evaporation rate, substrate temperature, bombarding gas, ion energy, ion current density etc.) has to balance between densification of the layers and the absorption. High quality coatings and multilayer interference systems with SiO2 as low index material can be deposited by various PVD technologies including reactive e-beam evaporation (RE), ion assisted deposition (IAD) and plasma ion assisted deposition (PIAD). In order to improve the degradation stability of dielectric mirrors for the use in UV - Free Electron Laser optical cavities a comparative study of the properties of SiO2, Al2O3, and HfO2 single layers was performed which was addressed to grow very dense films with minimum absorption in the spectral range from 200 nm to 300 nm. The films have been deposited by low loss reactive electron beam evaporation, by ion assisted deposition using a Mark II ion source, and by plasma ion assisted deposition using the APS source. Optical and structural properties of the samples have been studied by spectral photometry, infrared spectroscopy, x-ray diffraction and - reflectometry, as well as by investigation of the surface morphology. The interaction of UV radiation with photon energies close to the band gap was studied. For HfO2 single layer, LIDT at 248 nm were dete rmined in the 1-on-1 and the 1000-on-1 test mode in dependence on the deposition technology and the film thickness.