ture that is a mixture of small grains with high dislocation density and large grains with low dislocation density, to a microstructure with large grains and low dislocation density.

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Defects and morphological changes in nanothin Cu films on polycrystalline Mo analyzed by thermal helium desorption spectrometry

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Thermal helium desorption spectrometry ͑THDS͒ has been used for the investigation of defects and thermal stability of thin Cu films ͑5 -200 Å͒ deposited on a polycrystalline Mo substrate in ultrahigh vacuum. These films are metastable at room temperature. On heating, the films transform into islands, giving rise to a relatively broad peak in the helium desorption spectra. The temperature of this island formation is dependent on film thickness, being 417 K for 10 Å and 1100 K for a 200 Å film. The activation energy for island formation was found to be 0.3± 0.1 eV for 75 Å film. Grain boundaries have a strong effect on island formation. The defect concentration in the as-deposited films is ϳ5 ϫ 10 −4 , for films thicker than 50 Å and more for thinner films. Helium release from monovacancies was identified in the case of a 200 Å film. Helium release was also seen during sublimation of the Cu film ͑ϳ1350 K͒. Overlayer experiments were used to identify helium trapped close to the film surface. An increase of the substrate temperature during deposition resulted in a film that had already formed islands. Argon-ion assistance ͑250 eV͒ during film deposition with an ion/atom ratio of ϳ0.1 resulted in a significant enhancement of helium trapping in the films. The argon concentration in the films was found to be 10 −3 . The temperature of island formation was increased due to argon-ion assistance. The helium and argon desorption spectra are found to be similar, which is due to most of the helium becoming trapped in the defects created by the argon beam. The role of the Mo surface in affecting the defects at the film-substrate interface is investigated. The effect of variation of helium fluence and helium implantation energy is also considered. The present THDS results of Cu/poly-Mo are compared to those of Cu/ Mo͑100͒ and Cu/ Mo͑100͒ reported earlier.

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