X-ray reflectivity investigation of near-surface density changes induced in Al-Al multilayers by high-current ion beam bombardment

Alternating Al–Au multilayers (typical thickness of each layer 150 nm) were deposited on polished glassy carbon substrates by evaporation under high-vacuum conditions at 278 K and subsequently interdiffused with high-current 2.0 MeV 4He+ ions. After ion beam bombardment, non-destructive X-ray reflectometry measurements reveal a significant decrease of the density in the near-surface region from 19 to 12 g/cm3. This change in density is caused by the Al–Au interdiffusion during ion beam bombardment, as measured with RBS and X-ray diffraction. Based on the advantage of X-ray reflectometry of no specific sample preparation, detailed integral information of the surface roughness is achieved, additionally. For example, the surface roughness of the interdiffused Al–Au layers increases from 3.1 to 4.1 nm accompanied by the appearance of a gradient layer at the surface that even increases in thickness after irradiation. In addition, the density of this gradient layer decreased from the as-deposited to the irradiated state.