Defects and morphological changes in nanothin Cu films on Mo(100) studied by thermal helium desorption spectrometry

Abstract

Thermal helium desorption spectrometry has been used for a comprehensive experimental investigation of vacancy-related defects and thermal stability of nanothin Cu films (10–300Å) deposited on a Mo(100) substrate at 300K in UHV. In the helium desorption spectra (300–2000K), a peak is observed that shows that the Cu films are highly unstable. Postdeposition annealing treatments show that this peak is due to the thermally-induced island formation of the copper films. This process is strongly dependent on film thickness; its temperature ranging from 429K for a 10-Å film to 1312K for a 300-Å film, which is not simply due to the considerable change of stress with film thickness. Between 70 and 75Å, there is a distinct narrowing of the peak shape, indicating a change in the kinetics. For a 100-Å film lying in the large thickness range, an activation energy E=1.2±0.1eV was determined. The island-formation process is defect-mediated, as suggested by the fact that the temperature is lowered by an increase of the helium implantation energy and fluence. The island formation during deposition at an elevated substrate temperature occurs at a lower temperature than during postdeposition annealing. The helium released from monovacancies (E=1.8±0.2eV) and vacancy clusters in the Cu films and during high-temperature evaporation of the Cu films from the Mo substrate (Esubl=3.5±0.5eV) is also identified. The x-ray diffraction measurements on 60 and 200-Å films show that they are both fcc Cu(100) films with identical macrostrains; the 200-Å film has a sharper texture than the 60-Å film. The orientation relation between the film and the substrate was found to be (100)[100]fccCu‖(100)[110]bccMo.