In Situ Study of Titanium Film Growth On Different Substrates

Abstract

The chemical and microstructural properties of a surface have a strong influence on the growth mode and the morphology of a film evaporated onto this interface. Changes in the growth stress of thin titanium films, measured in situ by a cantilever beam technique, evaporated under UHV-conditions are used to monitor the chemical and microstructural properties of a substrate surface. The starting substrate film used in this study was a quasi single-crystalline TiO₂-film (d=50 nm) prepared by reactive evaporation of titanium in an oxygen atmosphere and subsequent annealing (20 min, 400°C). The Ti-growth stress on this substrate is compressive up to monolayer coverage and tensile at higher film thickness, which is interpreted to indicate a strong interaction between TiO₂ and the arriving Ti atoms at the interface during monolayer formation and strained (tensile) layer epitaxy at higher film thickness. In a second series of experiments the TiO₂-film was covered with Al-overlayers of varying thickness. Due to oxygen interdiffusion from the TiO₂-film an amorphous Al-oxide layer is formed at the interface eliminating the high degree of order of the substrate TiO₂-film. On this amorphous substrate the stress vs. thickness curve of the Ti-film, in terms of our stress model, is interpreted to indicate island formation and growth of a polycrystalline Ti-film. At Al-layer thicknesses above about 3 nm the Al-interface becomes metallic. The structure of this Al-surface depends on the film thickness and substrate temperature during its deposition. During deposition of the first Ti-monolayer on metallic Al a large incremental tensile stress (up to 45 GPa) is measured. The magnitude of this tensile stress is closely related to the surface microstructure of the Al substrate. The surface roughness deduced from the tensile interface stress is compared with the surface roughness measured by AFM.

For comparison, analogous experiments were made with Al₂O₃/Al substrate bilayers. The results of these experiments qualitatively agree with those on the TiO₂/Al-substrate. The general shape of the stress vs. thickness curve is comparable, however quantitative differences are interpreted to be due to differences in the structure and/or chemical composition of the substrate Al-film.