HELIUM-ION INDUCED EROSION OF CALCIUM-FLUORIDE THIN-FILMS

Calcium fluoride thin films of 17 nm and 105 nm have been evaporated onto a Be substrate in ultrahigh vacuum and subsequently subjected to irradiation at room temperature by 250 keV and 280 keV He+ ions. Periodically during the irradiations, the films' compositions and thicknesses were evaluated by time-of- flight medium energy backscattering spectrometry (MEBS) also using He+ ions. A total fluence of 2.0 × 1017 He+/cm2 was found to remove 82% of the fluorine from the thinner foil but to leave the areal density of Ca undiminished. On the thicker foil, a fluence of 2.3 × 1017 He+/cm2 removed 38% of the fluorine. More significantly, the F concentration was found to diminish uniformly throughout the volume of the film as the irradiation progressed. In each case, the initial desorption yield of fluorine atoms was found to be 0.7-0.8 per beam ion. © 1990.