Molecular beam deposition of fluorides
A new technique for the fabrication of optical interference layers is being introduced. MgF2, and NdF3, have been evaporated by molecular beam deposition (MBD) in an ultrahigh vacuum (UHV) system. The fluoride films are used in optical interference coatings for optical components of high power excimer lasers. The growth conditions were varied by changing the remaining gas composition, variation of the substrate temperature from 5 deg C to 30 deg C, and interruption of grain growth. The optical losses of the films were measured by transmission spectroscopy and laser calorimetry, and the film morphology was investigated by transmission electron microscopy (TEM). It was shown, that a stratification of very thin (1 nm - 10 nm) layers of two different fluorides can avoid the growth of large grains and wide columns, and gives rise to smoother surfaces. The packing density of MBD-films is higher in comparsion with fluoride films deposited under conventional high vacuum (HV) conditions.