IN-SITU STUDY OF THE GROWTH STRESS AND STIFFNESS OF ALUMINA THIN FILMS DURING VAPOR DEPOSITION

The evolution of the growth stress and stiffness of amorphous alumina thin films was studied in-situ during electron-beam evaporation on Si and sapphire substrates in the temperature range 170-400°C. The curvature of a cantilevered substrate was measured from the spacings between multiple laser reflections. In the initial stage of the deposition, a tensile growth stress develops immediately and has a constant value from the onset of deposition. No transient regime is observed. The value of the tensile stress decreases with increasing deposition temperature. The microstructure results from island growth, but the differences in tensile stress with temperature can not be explained by differences in island size according to the Nix-Hoffman coalescence mechanism, nor by stoichiometry or density, which remain the same. No stress change occurs during annealing at the growth temperature following deposition, reflecting the absence of changes in microstructure or surface structure. All films deposited at 3 Å/s, and those thinner than 0.30 mm deposited at 6 Å/s, had a biaxial modulus of 200 GPa, which is 0.4 times that of polycrystalline corundum. The greater compliance is attributed to a combination of structural (atomic coordination) and microstructural (17% closed porosity) effects. In films deposited at 6 Å/s, the biaxial modulus decreased continuously for thicknesses greater than 0.30 mm. This is attributed to the development of partially interconnected, open cylindrical pores, to an overall porosity of 27%, as determined by adsorption porosimetry. The ratio of the average growth stress to the average biaxial modulus is similar in all films, both with constant and with variable growth stresses, and only depends on temperature. A constant tensile growth strain may therefore be a fundamental characteristic of the deposition process.