Smoothing, compaction, mixing, sputtering and dewetting of semimetal- and halogenide-coatings by swift heavy ion irradiation

We have investigated the response of thin Bi, BaF2 and CaF2 films (100–150 nm) deposited onto Si and SiO2 to the bombardment with swift heavy ions (230 MeV Xe, 350 MeV, 600 MeV Au) at 80 K. The morphological and compositional properties of the films were characterized by means of Rutherford backscattering spectrometry, atomic force microscopy, scanning electron microscopy and mechanical surface profilometry. The as-deposited films showed rough surfaces and an enhanced porosity. Upon irradiation compaction of the films and smoothing of the surfaces occurs. The halogenide films on SiO2 showed a distinct interface mixing, while the halogenide/Si interface did not. In case of the Bi films, no mixing was observed with both substrates. Besides smoothing, compaction and interface mixing also sputtering of the halogenides takes place with sputtering yields of about 2500 for BaF2 and 1000 for CaF2. The ratio of 2.5 appearing for the sputter yields was also found for mixing and smoothing, which occur about 2.5 times faster for BaF2 as compared to CaF2. Astonishingly, perforation of the fluoride films was found at very high fluences, indicating dewetting by irradiation induced plastic or viscous flow of the coating, driven by capillary forces, as recently reported also for oxide coatings.