Evolution of the growth stress, stiffness, and microstructure of alumina thin films during vapor deposition

Abstract

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 of 170–400 °C. The curvature of a cantilevered substrate was measured from the spacings between multiple laser reflections. Tensile growth stress developed from the start of deposition. For a deposition rate of 3 Å/s, its magnitude decreased with increasing deposition temperature in the thickness range of 0.1–0.7 μm, from 540±97 MPa at 170 °C to 215±15 MPa at 400 °C. Ex situ analysis by atomic force microscopy (AFM) showed an island growth mode, with average radii at 170 and 400 °C of, respectively, 34±5 and 44±4 nm. For all thicknesses between 0.2 and 0.6 μm, the O/Al ratio was 1.51±0.03 and the density 3.15±0.27 g/cm3. No change in stress was observed during isothermal annealing after deposition. Tensile stress, therefore, cannot be explained by the Nix–Hoffman coalescence mechanism, changes in stoichiometry or density, or microstructural changes such as island growth. A constant biaxial modulus of 197±30 GPa was measured at 400 °C up to a thickness of 1.17 μm when depositing at 3 Å/s. This value was identical to that of films thinner than 0.30 μm deposited at an average rate of 6 Å/s at 400 °C. Above 0.30 μm, the modulus of these films decreased continuously to a value of 50±8 GPa at a final thickness of 2.05 μm. The decrease in modulus is attributed to the presence of open, cylindrical pores with a volume fraction of 27% at the final thickness, determined by adsorption porosimetry. The final average growth stress at 3 and 6 Å/s was, respectively, 171 and 40 MPa; the ratio is similar to that of the biaxial moduli. For films deposited at 6 Å/s, the thickness dependence of the growth stress and biaxial modulus was also similar. A constant ratio of stress to modulus, independent of film thickness, was found at other deposition temperatures as well. This suggests that at a given temperature, a constant strain is generated throughout the growth of amorphous alumina films, up to a thickness of 3 μm. New analytical expressions were derived to quantify the effect of surface roughening on the reflectance signal of a substrate/film/ambient optical system. Using the measured intensity of the laser reflections in the in situ curvature measurement, a constant root-mean-square roughness was found for deposition at 170 °C. At 400 °C, the surface roughness increased linearly with the thickness, in qualitative agreement with ex situ AFM measurements.