Properties of optical thin films and coatings prepared by reactive electron-beam deposition with and without ion bombardments

The structural, microstructural, optical, and mechanical properties of MgF₂, SiO₂, and TiO₂ films prepared by reactive electron-beam deposition (EBD) and reactive ion-assisted deposition (IAD) were systematically investigated using an x-ray diffractometer (XRD), a transmission electron microscope, a spectrophotometer, and a microhardness tester, respectively. A mixture gas of pure argon and pure oxygen was used as the reactant. The mean ion energy was about 90 eV for the IAD process. Results showed that the preferred orientation, refractive index, and hardness of the films were strongly influenced by ion bombardments, although the variations of the phases and grain sizes were insignificant. With the fixed x-ray incident angle of 2 deg, the measured preferred orientation of polycrystalline MgF₂ films deposited on unheated glass substrate by IAD was [110], which was consistent with the powder XRD pattern of MgF₂, whereas that of the film deposited by EBD at a substrate temperature of 280°C was [111]. The refractive index and hardness of the films deposited by IAD were always higher than those of the respective films deposited by EBD at a substrate temperature of 280°C, which were due to the higher packing density caused by energetic ion bombardments. A 24-layer near-IR cutoff filter of alternating SiO₂ and TiO₂ layers prepared by IAD was more efficient on near-IR isolation and more thermally stable than that prepared by EBD. The optical thickness variation for the filter prepared by IAD over a substrate holder radius of 30 cm was less than 1%.